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

I would remove the "movement" of troops without a hero; it's just absurd.

I believe this isn't absurd, but rather a unique feature of the game. It's an important feature that is sorely lacking in the third and fifth installments. For example, you're exploring the map in the third game. You encounter 400 Minotaur Kings. And – what a surprise! They decide to join you. And then you frantically think – should you discard the Harpies (and lose the ability to quickly and without losses deal with small groups of neutrals), or discard the Angels, which you collected in five Conservatories one after another, or perhaps get rid of the five hundred Pit Lords that joined you a month ago... In the end, you discard one of the useful, necessary units, and take these damned Minotaurs. Equally useful, necessary, and powerful.
The question is: why should you discard anyone at all? Where is the logic, where is the playability, where is the meaning, in the end? It turns out that if a hero with diplomacy skills has gathered an army of his city (all seven levels of creatures), he must prepare in advance to discard various parts of his army during the game? That is, the money spent on Screechers is doomed to be wasted. Or should he leave the least useful creatures in the city in advance (which reduces the "meat" and lowers the chance of neutrals joining him) in the hope of replenishing the Screecher squad with other races.
The movement of troops without a hero, their gathering of resources, the setting up of "guards" designed to keep the fog of war open on the borders of the kingdom – this is correct and logical. After all, in the fourth game, troops without a hero cannot capture a mine, nor can they claim ownership of another city (even if they kill all its guards) – and in the end, in the fourth game, troops are an order of magnitude weaker than heroes. No one will ever FIGHT with hero-less troops. But the problems with the redistribution of troops, discarding parts of your army, the hassle of recruiting creatures in external buildings (when in the third game we recruit Golems and the hero spends a week dragging them to the castle) can be forgotten forever.