I agree that many creatures from Heroes of Might and Magic III are missing in the fourth installment (perhaps that's why it's not very... popular?). For example, liches and gorgons, as mentioned above. On the other hand, how would that affect the balance?
Regarding artifacts... It would have been possible to create artifacts that increase each skill by 50% or more, swords that give +100 to skills, etc. Again, how would that affect the balance?
The game itself turned out to be complete, so it doesn't need anything extra (except maybe liches :) ).