I love AHD as it stands, and I don't expect Vicky 3 for a loooong time, but there are definitely some things I'd like to see changed.
A National Manpower System: Long prior to the era covered by HOI, numerous nations were moving towards a national manpower system that put soldiers from all over the nation together in units. Several actually started long beforehand. I'd like to see it start with a technological innovation or an invention, be started with a major decision that most great and secondary powers would undertake, and be followed by a multi-year organizational and leadership malus as the nation moves in that direction. Reserve units should stay regional. Units being recruited from a national manpower system would spawn in the national capital unless a National Focus was used to move the spawn point for such units elsewhere.
A Rework of Military Structure: This is actually two ideas.
1) I'd like to see the number of individual units that both a nation and province can support have a hard limit, similar to the supply limit. This limit can be modified by wartime mobilization, tech research, etc. but effectively the point is to keep most nations from having more than 30 or so independent unit groupings, both for armies and navies.
2) Unit size changes. I think that that the base unit size should either be 1000, or that primary units like infantry/cav/dragoons etc should be 3000 and artillery,air,tank,engineer etc should be 1000 in the same grain as HOI 3.
Assimilation Rework: In my USA games, I've seen French and South German immigrants assimilate to Afro American. It's just not that simple, for obvious reasons. While I understand that it is a touchy subject, I think that culture groups should be under a further "ethnic group" (call it whatever you'd like), such as the basic "European Caucasian", "African", "Indian/South Asian", "SE Asian", etc. Some of these already exist as culture groups. Effectively, groups from one ethnicity shouldn't assimilate to a culture group under a different ethnicity unless that ethnicity makes up an overwhelming percentage of the local population (>90%). Not because they can't, but because during this time period, they didn't.
Effectively, I think that immigrant or recently conquered populations should assimilate to the primary culture of their new country, OR to the predominant accepted culture of the province if the percentage of persons of that that accepted culture is greater than the percentage of the persons of the primary culture OR they should become an unaccepted culture or not assimilate if the overwhelmingly predominant culture (>75%) in a province is an unaacepted culture. I'm not sure if that's how it already works, but that's how it should work. 10 French people moving to an 80% Polish Warsaw should become probably become Polish eventually, even if the province owner doesn't accept Polish culture.
Other thoughts:
- I like the corporations concept, as in the last 40-50 years of the game they are a huge economic driver. Factories in your own country having to compete against eachother would give rise to a stronger push for either state capitalism or socialism/communism from people tired of being slaves or getting laid off/fired when their factories go under. In this same grain, I also think that the population's awareness of wealth inequality (based on their wealth vs. average wealth of rich strata in their region/nationally, with this awareness being influenced by consciousness, literacy, etc) could also drive pushes for social and political reforms or even push pops into the communist/socialist/fascist camps in the lategame.
- Parallel research, similarly to what has been previously mentioned. Research should be undertaken by capitalists or other pops, and subsidized similar to factory output. I think a great way to do this would be to introduce a new poptype, the intellectual currently used by some mods. A National focus could be used to boost a state's research. With this in play, the tech tree can be massively expanded. Theoretical research can unlock work on practical inventions.
- Universities: In a fork of the research thought, I think that educational institutions should play a part in research. They can be established by the government or by private citizens depending on reforms, government/party types and policies, etc. The openness of higher education to the middle class can also become an "issue" in the later game, and nations which push for public schools open to all can get great boosts to leadership and lategame research.
There's a lot more that can go here, I'm saving it for when they ask for thoughts for the next Vicky expansion. I agree with others that they can throw at least one more small expansion on before it's fully milked.