Honestly while nazgul-esque swooping attacks sounds awesome, I have no idea how to balance it. Essentially making them immune to melee and therefore hard-countering them. So if you find yourself in a position where you only have melee units left vs a flying unit (which will happen, especially in multiplayer) you have lost.
Really hope flying monsters are hardcapped at 1. Or else I forsee multiplayer armies consisting entirely of flyers.
Better yet. Re-introduce the system from Med 2 where units cost more the more you spam them. It was great for balancing, no idea why they got rid of it.