Well, I have a whole list of ideas concerning possible improvements and enhancements for Victoria 2 compared to first part.
I split them into several parts.
MAPS
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1. Remove the physical map, or at least make it useful for something. In normal Vicky it was pointless - there was no single thing it was good for (because it was fancy and unclear it couldn't be even used to see what terrain was there in provinces). Either provide a solid, informative terrain map (like in HoI2), or do away with it entirely.
2. It would be great to see maps with economic/demographic informations. Like population densities, production total, production per capita, profile of production, etc. Like "every economic/demographic map you could find in a proper geographic atlas". I was always lacking those maps, while not using the "phsical map" at all. Also, if done this way, there has to be "unemployment" map, so you could actually see where there are not enough factories or underdeveloped RGOs.
3. Provinces should be made a lot more uniform in sizes. It was quite silly how a siberian province was as large as a quarter of Europe that contained 100 provinces. Especially so since the distance was measured in provinces. And building transsiberian railway was very cheap. Cheaper than building railway from Berlin to Moscow. After all, it was "closer".
4. States should be made a bit more uniform in sizes. Not so much as provinces, but a little bit. It was strange seeing "states" such as Boston or other miniscule 1-province states cramped on the east coast, while the same "state" in northern Canada was half the size of whole USA.
5. It should be visible or checkable if given state is colony or not ! It was silly that the only way I know in Victoria to learn whether given state is colony or not was to try attacking it in colonial war. In colonial war you couldn't plan anything, because you could only wonder "would I be able to attack here or not ?". And then of course it would turn out that Freetown in Africa is not a colony but a proper english city, basically on the same rights as London.
ECONOMY
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1. Goods reserves should be state-owned reserves. But in normal Vicky, the stuff produced in your own country magically and free of any charge go to this reserves. Since the pepople that actually produced these things in factories get money for that, you should be paying for these goods ! On the other hand, selling stuff from the reserves should directly earn you money (after all, it was the government that owned those goods). Aside from that, when you set goods to "buy" or "sell", it should be also possible to set whether to buy/sell within the country, or use global market. This way it would be possible to just (for example) set "small arms" to "buy" and "internal market", so that your national reserves are buying arms from home producers only. Or in the times of crisis, set "grain" to "sell" and "internal market" so that your people have access to grain even if there was some drought or other problem with grain production (setting it to internal market would assure that you won't help for example those pesky neighbors - let them starve !)
2. Imports in Victoria are not imports. It shows STATE imports only. It should be showing country imports, meaning all that your pops buy from abroad (and pay tariffs from). Exports in Victoria were not exports, but more like "market volume" (from what I seen) - the stuff that your own pops bought were also included in exports. That's wrong. It should be really telling what is sold to OUTSIDE of the country. Of course the indicator for "internal market volume" showing how much stuff is being traded between RGOs, factories and pops would be also very very useful.
Additionally, it would be REALLY good to be able to look up the detailed reports on exports/imports/internal market, i.e. what goods are being imported/exported/traded internally and in what quantities. It could enormously help to decide what factories to build or what RGOs to prioritise. You wouldn't have to wonder anymore "is my grain production enough to feed all my people, or am I importing grain ? or maybe i'm actually exporting massive amounts ?" or "what goods are actually in short supply and what factories are needed ?"
I always did that, for example not having a clue how much clothing for example was used by the country as a whole (by pops).
3. Factory supply is another slightly illogical thing in Victoria. If you didn't have home supply of certain goods, then STATE had to buy materials for the factory, while getting NO income from that. Factories should be able to buy supplies on world market, just like pops can. After all, it's those pops that "run" the factory and get any income out of it. As a state you only get income out of the pops through taxes, not from factories themselves.
4. There could be also factory ownership: private or state. If it was private, then factory is entirely run by pops (or more precisely, by certain capitalist pop), it should be able to get supplies from wherever it wants and sell it as it wishes. If the factory was state property then the income share that normally would go to capitalists controlling the factory would go to your budget, as divident (might turn out negative if factory is running in the red). Still it would be factory's directors responsible for buying supplies and selling products, not you (the state) directly, so aside from getting a divident you would not get any other freebies nor obligations (still factory buys supplies itself, and you have to buy products if you want them in reserves, but the divident will offset the cost considerably.
5. If the distinction betwen private (capitalist-owned) and state (government owned) factories is made, some of them may be outright disallowed in certain economic organizations. So for example State Property economy would disallow all private factories, while Laissez-faire would disallow state owned factories. If the economy changed, then the old, not fitting factories would have to be "converted" (bought by the state or sold to capitalists), although maybe not instantly. For sure no new private factories could be built under State Property economy, nor any state factories could be constructed under Laissez-faire economy. Maybe in case of Laissez-faire, few special types of factories would be allowed to be state property, namely military production factories, like small arms, artillery and tanks - the factories that produce goods not used in civilian economy at all, only useful for military.
DEMOGRAPHICS
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1. Immigration/emigration should be made more realistic. It's rather strange that in regular Victoria tons of people prefer to migrate to Alaska and sit there unemployed, rather than moving where they actually could get a job !
They should pay more attention to job than "gold somewhere in the state" that they will never touch. Moreover, there should be larger degree of randomness as to where people migrate to. I got the feeling that it's very deterministic in Victoria - huge piles of immigrants in one, eventually two states, while not a SINGLE person in others. People have in reality so many reasons and considerations that they NEVER do things so deterministically. To model that, an appropriate noise in decision-making should be applied.
2. Make pops effectiveness more tied to their size, so that 500 people in 5 different pops are not more efficient than 1 pop or 10000 people. While pops are a very good idea, the effectiveness should be linear (or close to) with its size to avoid silliness. Moreover, RGOs and factories should be limited in total PEOPLE working, not POP number. So let's say factory level 1 could have 10000 people working in, regardless how many different POPs would that be. If everything tied to POP would be scaled with its size, then the behavior of whole game will be more believable and realistic. If some leveling factor for overpopulated countries/provinces/states is needed, so that they don't dominate the game completely, the effectivness of employment in factories/RGOs could be sublinear. Meaning that factory or farm with 1000 workers does not produce 10 times more stuff than the one with 100 workers, but for example 5 times more goods. The exact sublinear dependence is up to decide. This will also have a side-effect of making overpopulated factories/RGOs and therefore overpopulated provinces less attractive, since income per capita will be lower.
[EDIT:]
3. Allow POPs to evolve by themselves. After all, the state didn't dictate who was who (and for sure not for democracies). The by-hand POP promotion to different types is not very realistic.
For example: The evolution from lower to middle classes would be dependant on education. After all, the promotion in Vicky was "Educate to Clerks" for example. So the higher the education slider, the more people actually manage to advance from being laborers to craftsmen, or from craftsmen to clerks. Depending on military policy, POPs should convert themselves into Soldiers or from Soldiers to end out better. Soldier POPs would be paid from "Defense" Budget, so the higher it is, the more people would decide to become soldiers because it would pay better than still mining coal (for example). In similar vein, if you reduced the slider enough, people would elect to cut timber rather than live in old, half-ruined barracks for bare subsistence wages.
The advancement to aristocracy would be nonexistent, because you really need to be born aristocrat to be one. So only growth and migration (though aristocrat might be veeery unlikely to migrate). And clerks rich enough might advance to capitalists, to start a business (see the point about private factories in Economy).
4. Disallow "by hand" POP promotion/demotions. Maybe except some autocratic regimes. After all, usually people decided themselves what to do (and usually went with the best they could). You could control the population composition to some degree through demand for their work. If there are lots of understaffed factories, naturally many people will decide to become craftsment. If the soldier wages are extraorbitant, even some good-earning clerks might decide it's a better life and become soldiers. Similarly aristocracts might decide that army career as officers are better for them in such a country.
Bottom line: no micromanaging what people do. Let them decide themselves, just allow player (the government) to set up some scheme of push and pulls and let the machine roll !
5. Similar to POP promotions/demotions, it should be the people that decide where to work, not player. Of course if you have state owned factories, you might elect to close them to force people to other jobs (just don't expect such people to love government = +MIL). The POPs would just usually take the best work they can find (with some noise added to take into account personal preferences, situations and other factors not covered by socio-economic model). Additionally, some POP issues might influence their decisions. Pacifist POPs would not become Soldiers even if they were starving as laborers, while Jingoists would not go back to coal mining unless really their necks were on the line (though it's more likely they would just increase in MIL and revolt).
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MILITARY
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1. The system of manpower in Victoria was not good one. You could exploit it recruiting many unit of certain culture, that the population of that culture could not really support. And the home province didn't count THAT much.
Maybe provide each state with separate manpower pool, and split units not by culture but by state ? All losses to the army would be inflicted appropriately among all soldier POPs in its home state. And it would be the status of the state (state/colony/whatever) that decide the possible troop qualities.
2. I expect already that the combat modifier system would be like HoI (meaning multiplicative) instead of additive. Additive produces baaad results.
I also noticed the troublesome behavior in Victoria, where single irregular division (of my satellite) was completely ruining the dig-in bonus, because it got averaged over my perfect 100 dig-in and 0 dig-in of irregulars running around for no reason. So extra division actually made the position weaker, which is not realistic.
POLITICS
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1. Changing the way you can grant statehood to states, so that Freetown doesn't become proper english city just like that. There seems to be little difference between REALLY home provinces, the core of the country, and some end-of-the-world state that just happened to have some immigrants of your culture. Maybe make further distinction between Homeland, States, Colonies and so on ? There should be some difference between London and Freetown in its status. Maybe give more weight to the cores ?
Like Statehood + Core = Homeland, Statehood + non Core = State, no Statehood + Core = Territory, no Statehood + non Core = Colony ?
Or some similar system that makes difference between realistically different levels of "belonging" to an empire.
2. More political options ! But I would take that for granted now. Victoria had really little you could do with other country other than declare war or trade through Open Negotiations. No demanding territories, no threats, no taking claims as war spoils, no demanding specific actions on the part of others. When you are running world-spanning empire, you need options to pressure opponents on different fronts. In Vicky it seems nothing goes on without a war.
3. Ability to see some information about other countries, other than just ranks in Prestige/Industry/Military. Who are their ruling party ? What are their ideologies ? You could technically check it all in Statistics, but why not give access to that in some more reasonable place.
Similarly, you could not even see the approximate population of provinces owned by others, or those unclaimed yet (you can see the culture pie chart, but not the total population, or at least estimate of that).
4. Ability to do some politics on the state-scale. For example inciting colonies to secede from their masters or raise militancy (thanks to POP system, a liberalist propaganda might work on liberals under nonliberal government).
TECHNOLOGY
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1. Reverse the situation with "Inventions" and Technologies. What I mean, the basic, core concepts like Romanticism or so just spread. The government doesn't have to actively pursue Revolution & Contrrevolution for the people to get that idea from other countries or by circumstances. It's the actual technologies that ARE developed by countries. So getting the "Idea" of a Barrel/Tank might trigger by chance after a heavy Verdun-style static battle, while actually developing technology to make them is up to governments. Not the other way around !
This would give much more realistic behaviors. The general ideas and notions spread by themselves. Of course Literacy could affect the rate at which ideas arrive in your country. Different Ideas could have different "infection" rates, sometimes conditional. For example: Military organization ideas might be spreading slowly. But should you have a war with another country, their military ideas might spread a lot easier to you, as the soldiers and commanders on the battlefield learn concepts of the opposite army, and the effective things they employ. On the other hand, Revolution & Contrrevolution might be like a plague, impossible to stop by any means, and quickly spreading throughout the world. Presence of socialist ideology among your pops might put that rate even higher.
On the other hand, the technologies themselves - knowledge How to Make a Tank, or How to Set Up Military would have to be actually developed by the governments.
The most powerful feature of this setup is that Ideas can appear in response to what happens. If there are opressed, poor people, then Revolution will come to their minds. If there are commanders seeing futility of charging through barbed wires, an armored fighting vehicle might pop into a mind of a brilliant one and so on.