Are you suggesting changes to a specific mod, or do you want a separate one?
As someone who has played Heroes 2 (and even tried the first one), Heroes 3, and Heroes 4, I have a certain impression of the "Necromancer" faction.
Yes, raising undead should have been limited in some way... but Necropolis is about skeletons.
And from a balance perspective: building those liches... and simply raising them - there's a difference.
Also, for this mod, I think it's necessary to allow raising "bone dragons" (from level 7 monsters) after level 20 of the character... But only level 7 troops. Levels 1-6 - skeletons.
And why did you ask? Did I write correctly where and what needs to be changed?