If you do give mobs/bosses additional attacks, the best place for it would be the +/++ versions of the dungeons. While increasing the health/damage of enemies/mobs is certainly a way to increased the difficulty, I think the enemy types and tactics changing is a much better way of doing it. In that regard, playing today made me think that the enemies having a chance of spawning with elemental weapon damage would be a great +/++ feature!
Not sure if it's a bug, or a resolution thing, but enemies health bars don't seem to ever fully empty visually, there's always at least a sliver of red.
Another thing I thought of as I played today was that a little sound effect/visual for clearing a wave of mobs. I can see the reason for it if you want to have the player explore the level to check that he got everyone, but I don't think letting the player know they wiped the wave is bad either. Honestly completely up to you.