I wouldn't change cooldowns myself, as it might create an imbalance between lone wolf/no lone wolf setting: as it's easier to keep lone wolf characters under CC.
That said, I would definitely increase enemy HP and dmg output by at least 50% on a very-hard and 100% on a nightmare setting (in comparison to the current 'hard' setting). Maybe gaining drastically less money and/or experience would make sense too?
I also wouldn't make any changes to the current difficulty settings, but rather just add new ones.
