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

Too easy and simplistic... this is the part of the game that could be improved, made more complex, and more diverse... isn't it?

No. In the first week, the resource shortage is significant, and you often have to go to great lengths to gather enough to build. Later, other priorities emerge. But even then, the game itself lasts only 3-5 weeks.
Besides, in theory, you can build and upgrade other castles to the maximum, so resources can remain relevant at all times. The problem is that no one plays like that online.