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

well, that's not bad.
but:
1) - it doesn't really affect the game.
2) - this is, in my opinion, a highlight of the game - it shouldn't be removed.
3) - definitely.
4) - partially true, but everyone is already used to it - it's not critical.
5) - is it even necessary? does anyone use the Archmage?
6) - a good idea.
7) - hmm... it's unlikely to be needed - it will create an imbalance - life will become unbeatable, races that have access to life magic will make it their main hero, and races that don't have access to it - well, it's obvious anyway.

and here I completely disagree

Because of this, Life magic becomes the weakest, as it is focused on supporting its own troops. This is an imbalance!

there is no imbalance - life is the most powerful magic. the question is rather - who knows how to play with it.