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

in my opinion, in Heroes 4, nature isn't entirely alive; it could have been done the same way as in Heroes 5.

No, Heroes 4 and Heroes 5 are different games, and they are very different. Nothing good will come of swapping components from one game with components from another.

a hand-to-hand combat school for barbarians...

There's no point in that. They already attack very well.