I would like to suggest that you should not focus on increasing the number of skills and abilities, especially the number of hero classes. This has probably become tiresome for many (my assumption). Because there is no need to multiply what already exists without any real need. It is better to focus on the following:
1) Perks, and in particular: Cold Steel, Reflection, Magic Drain, and Dark Regeneration. They are extremely useless in the vanilla game (although the idea itself is quite good). I would like to see them strengthened.
2) The ability to have multiple summoned creatures in battle at the same time (elementals). Multi-summoning, as in Heroes 3.
3) The ability to continue the battle with summoned creatures after all non-summoned creatures have been defeated. Again, just like in Heroes 3.
4) Fixing the specializations of Sorgal and Talanar. Otherwise, they are completely useless.
3 - This is practically impossible.
Point 4 seems to be the easiest to implement