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

Variety is definitely better. In Heroes 5, there's hardly any room to explore different spell choices. Look at Might and Magic 8, where there are also identical spells:
e.g., Flight (air magic & dragon abilities)
e.g., Charm (mind magic & vampire abilities)
But of course, too much is also bad.
e.g., in the game TES III: Morrowind, there are many spells that are identical.
e.g., Levitation and Wild Levitation, as well as 6 types of Shield spells.