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

It would be worth fixing at least the fact that resource mines themselves have no value, and guarding these mines is a greater reward than the mine itself... and after buying buildings, there's nowhere else to put the resources.
The development happens in an excessively... geometric progression, I guess.
In my opinion, the strategic component of the game is not developed at all.
I don't know... you get used to good things quickly, and you always want something better.

I think it's better to play by the established rules of the game, rather than inventing your own. When I first played on level 1 difficulty, I felt a strong shortage of resources for building and recruiting an army (I don't remember about the fourth game, but it was definitely the case in the third). Now, when I play on level 5, I don't feel that - on the contrary, over time I see an excess of resources (exactly in the geometric progression you mentioned). And this is thanks to the mines, as well as their rational use. In life, too, a state becomes richer with a properly established economy.
If you take the weekly income of a village, it comes out to 3500 - you can't even buy one level 4 creature with that. And you can only increase the resource once - the second time, it won't be there, you spend it - and you're back to having no money. And a constant source like a mine is exactly what is needed.