The tedious and unclear sections of Vicky need to be cleared up.
Definitely, this should be one of the baselines for a possible sequel.
Sphere of interest should have automated option. This is the country I want influence in, this is how important it is, discredit ban and expel foreign ambassadors as needed just keep me in control. Done. Constantly checking the screen to see how many points the other country has in Colombia or Egypt as you near the canal building is just frustrating. Fighting over German minors is worth pulling out the hair on your head.
I would argue that having it be too automated would be a bad thing, as it would be overly mindless to perform and could lead to stuff slipping though the cracks. Shifting the sphering controls to the diplomacy menus and organising them into groups, similar to how EU4 handles diplomacy, I feel would help. The sphering list in Victoria 2 is just horrible to navigate.
Then the pops being constantly independent and going into red. Soldiers should be pooled as a manpower. You can theoretically have hundreds of thousands of soldiers you can’t use because they’re under a thousand pop total in province. The pop changes and growth mean that you can build 50 brigades and then it grows to 70 but some of the brigades are red because that specific pop fell down in numbers forcing you to replace it. If there are enough Yankee pops let me get some Yankee brigades and keep them topped up.
This is a great idea, I don't know when having soldiers of different pops actually affected my army and pooling it as manpower, like every other Paradox game, makes forming an army much more simple. I would argue, for some balance, of making the manpower be just from your core population's soldiers, with options to produce troops from other culture's pops requiring you to form colonies or dominions, which would encore the emergence of Canada, Australia, New Zealand, South Africa and The Raj for the UK in most games, and having you control their armies unless you grant them the option to command their own, as many of the dominions of the UK were granted during WW1.
There is not enough difference between ideologies. The only real economic difference is state capitalism and others. All others do the same thing which is rely on capitalists to build. Let use plant ghost factories in interventionism; ie I want a steel plant in Kazan I place a proposal for it as interventionist and then capitalists can fund it rather than building a random plant in an unsuitable place.
The support of different reforms and government types is one of the bigger differences in Ideologies, with Liberals socialists and Communists supporting social reforms and Conservative, Reactionary and Fascist opposing them as being one example. Expanding internal politics would certainly help give more distinction between the different ideologies, as aside from the Communists and Fascists which change governments, the others are mechanically the same outside the Economic policies you mentioned.
One idea I see working to help assist with this would be giving players the option to build a party to suit their needs and working to convince their pops to elect them (in a democratic system). There should be some restrictions (to stop you building a Jingoistic, Planned Economy Liberal party, to get everything you want) but I'm not sure what restrictions would be fair. Just seems a better system then the current election events, which often leads to 4-5 different parties for each Ideology.
Clear up and explain the trade screen.
I think one of the devs mentioned they wanted to destroy the world market, so I don't see it returning in a future game in the way Victoria 2 has it.
I agree with your other points. They're well though out and I can't think of anything I could add to them that I haven't already said.