I should sit this thread out because it opens old wounds. They are both great games made for different audiences that happen to have a lot of overlap. Hoi3 was an amazingly rich, deep, complex (but flawed) WWII simulator. HoiIV is a less complex, but no less deep, sandbox game with a WWII setting.
I don't run a game company. So, I don't have to worry about resource management, product roll out, budgets, market interest, or changing gamer demographics. I can only speak for myself and what I would have liked to see (which I understand is completely selfish and unrealistic). Hoi3 was never finished. I would have like to see the continued development of that game. Each expansion that was release was an entirely new game that built upon the original game, we could have used a few more. I love equipment manufacture and division recruitment in HoiIV and many other features are greatly improved. However, there are a bunch of features that didn't make it to HoiIV that I miss. I know podcat has stated that systems in Hoi3 were buggy, couldn't be fixed, and needed to be reworked from the ground up. Luckily, in my fantasy, I do not have to worry about that.
One of the best features about Hoi3 was that it was overwhelmingly complex, but you could automate the parts of the game that disinterested you. Not interested in espionage, automate it and the game will handle it for you. Not interested in trade, automate it and the game will handle trade for you. Not interested in assigning leaders to each division, automate it and the game will handle it (badly) for you. Instead of trashing divisional leaders altogether, they could have worked on the routine of assigning leaders by the AI so that it made more intelligent assignments. It knew enough to assign panzer leaders to armored divisions and mountaineers to mountain divisions. So, it could be done, then add periodic reassignment of leaders by the AI. I know that is a great amount of work and a complex system for something most players would just hand over to the AI and forget, but it goes along with my theory of the player being able to decide where he wants to micro and where he wants to automate. For the Soviet Union, microing divisional leaders would be overwhelming even for those with OCD, but for Argentina it could be cool to follow the carreer of Lt. General Juan Peron.
I liked the useless complexity of Hoi3, because it added flavor which I think is lacking in HoiIV at the moment.