RTF is a global modification that replaces all towns in the game.

The idea is simply that living dragons gain strength from their connection with their gods, but skeletal dragons do not. Also, I think it would be more interesting to place a different creature on level 7 instead of dragons.

Of course, it's interesting, but then maybe a replacement should be found for the dragon? Either way, even though the skeletal dragon is no longer imbued with the power of the gods, it is sustained by dark and powerful magic. The creation of these dragons is considered by necromancers to be the pinnacle of their skill (like the creation of titans by mages)! However, I don't have anything to offer as a replacement for it yet... except maybe black riders?

By the way, I'm also confused by the fact that one of the Lich's upgrades is a necromancer (who is not necessarily a Lich). Maybe it would be better to make it a necromancer hero, and the unit – an archlich? Or should we change the level 5 unit altogether? For example: black sorcerer => necromancer => void mage? Or sorcerer => shadow adept => void mage?

In my opinion: if all Necropolis heroes are fallen angels, it will negate their diversity. And in general, a class is a class because it denotes a CLASS, not a race.