Same here. I'd like to see more character among the heroes by having a broader array of powers and abilities, and then having their entire army take on some of the qualities of the heroes in the group.
Just giving buffs, like movement speed, is a good start--but maybe also certain static abilities or elemental resistances across all units in the army the hero occupies. One might then choose the composition of units around particilar heroes based on their power(s). That would offer interesting strategic options.
Guildar, tech knowledge, and arcane knowledge per turn is nice and I like that, but buildings already do that, and there are a lot more abilities that champions could offer.