I had a very interesting idea regarding a mod for Heroes 5.

exactly, that's the idea, and as the skill level increases, also increase the level of the creatures that the skill will affect (like with Inferno), and increase the duration, most likely 2-3-4-5 turns.

Added 1 minute ago
and the question is, what secondary abilities to take, and most likely, the skill tree will need to be completely reworked for them.