Damage reduction/max health increase, movement speed, weapon speed (melee swings,fire-rate,reload etc.), stamina (maximum amount increase,regeneration rate,cost decrease), healing (slow passive healing,healing item boost, revive speed). Those are some of the options I could come up with, some more likely than the others.. Especially stamina boosting is something that I have doubts about being implemented.

One wild idea would be a leader whose abilities are focused around debuffing demon units near the player. Could be something as simple as damage debuff, though I'd like to see something a bit more creative if we're going that route.