I'd love for Victoria 3 to expend on the economy. Yes, Victoria 2 has a better economic simulation then the other paradox games, but it still behaves rather weird at times.
What always felt very strange to me is that there is no such thing as the owner of a factory. It doesn't matter, if you, or the capitalists build a factory, the only thing that matters for your degree of control is which party is in power.
In Victoria 3, you could distinguish between privately owned and government owned factories, with government owned ones being way less efficient, but giving you more control and all the revenue (whilst privately owned ones give you only what you tax them).
You could also privatise your factories, giving you a nice sum of money and handing them of to private owners or nationalise them, either by buying it from the hands of their current owners, or just confiscating them (if your party allows it), with the latter massively hurting your investment rating, making your (and foreign) capitalists way less likely to build new factories in your country.
The economy could also be based more on companies then factories directly. There would also be the collective term "smaller companies" so you don't have to simulate thousands of companies
You could also expand on trade logistics, allowing you to build up (or have the capitalists) build ports and specific railway connections (instead of just the generic infrastructure level). You would have realistically simulated trade routes (with your maximum trade volume being limited by the number and level of trade routes) and get the satisfaction of having other countries depend on your ports for their imports and then embargoing them.
The tech system also has room for expansion . It feels strange that some of those techs are for you to research. In Victoria 3 many technologies would research by themselves , however, you can still provide funding to increase the research speed, or start a public research project on them.
Similar to how institutions work in EU4, you'd have technologies spread. Meaning that many technologies will not directly affect all your provinces, but will have to spread slowly (with the most determining factor being literacy). Techs could also spread into other countries. You could be given a few laws that trade higher research speed for slower technology spread or higher research speed for an increased chance of your technology spreading to other countries.
Another thing that could be added would be patents. You could either ban patents (which decreases research speed), or allow them. If you allow them, each new technology that you research (unless that technology already spread into your country from another one) will create a new patent. If a technology spreads into your country from another country, the owner of the patent in that country will also get that patent. Newly created patens will be assigned an owner depending on certain factors. The owner can be either the government, one of your companies, smaller companies or foreign companies. If the owner is not you or smaller companies, it will increase monopolization, with high levels of monopolization hurting your research speed, spread and factory efficiency. Patents can slowly move by themselves, thereby decreasing monopolization (unless they move to a company that owns many patents, then monopolization will be increased). To counteract monopolization, you can buy patents from larger companies or pass certain laws. With patents that you own, there is a variety of things you can do. You can either sell them to larger or smaller companies (smaller companies will pay less but don't increase monopolization), sell them to a foreign countries (you will still keep that patent in your country, it's just that the other country will instantly research that technology and get a patent for their country), or you can publish them. Publishing them will decrease monopolization and give a small research bonus, however, it will also remove that patent, meaning you can no longer do anything else with it. If a patent is owned by a company, it will increase the spread of that technology into states with other factories owned by that company (even if they are in another country). If that company does not own any factories in another country, technology spread into other countries will be reduced.
The way you research could also be changed. Instead of literacy being the main determining factory (being able to read isn't really enough to invent more complex things), it will be based more on companies and research institutions (research institutions are similar to companies, with the difference that they don't own factories and that they can be either state funded or private (in the later case they can create patents, which you might not want)). Companies (also "smaller companies") and research institutions will automatically assign themselves to technologies and start researching them (If a state funded technology is researching a technology, it will increase the chance that the government will get the patent for that technology). You can then either give some funding to that technology (that will simply increase the rate at which it is researched), or start a public research project. If you start a private research project, you will have to provide a lot of funding, as well as assign some companies and research institutions (both public and private) to them. When a public research project completes, the government will be guaranteed to get the patent for it. The speed at which a research project completes will be based on a number of factors: How many companies and research institutions are assigned to that technology, how much funding is provided and how good the assigned companies are at researching that technology (that is a factor of how similar that project is to what the company is producing, the size of the company, how many other projects the company is researching as well as how qualified the companies employees are).