What would you change in Heroes of Might and Magic IV?

Well, the idea of tying the town screen to the environment is actually quite good, especially since it was already being considered.

It wasn't just being considered; it was even present in some games. In Master of Orion 2, for example, the appearance of a colony depended on the type of planet it was colonizing. However, the building models were the same for everyone, so the implementation of this aspect was much simpler.

I'm not too bothered by the mismatch between the environment outside and inside the town; I get a lot of aesthetic pleasure from the town screens in parts 2, 3, and 5, and I don't really think about how inappropriate those landscapes are in the town when compared to the adventure map.

I agree. And with the Dungeon, there aren't even any problems in this regard – you just have to imagine that you have a fortress-mountain. Besides, even in the Necropolis, the "native" dirt still contrasts with the dead soil of the town screen. What really bothers me is that the Necromancer's native land isn't a wasteland, and the Sorcerer's isn't a desert. It was different in role-playing games!

In Heroes 2, you need to draw a full background with all the details, and the building sprites need to match it; you need a separate sprite for each background.

To be fair, they didn't even bother to make several models for mines and some other buildings! But yes, in the context of towns, this would require a lot of effort if you don't have a game with a very simplified graphics style. And you can always leave it to the fans, as you can see from the mods with town screens for the third game.