I agree that something similar to cliffracers...yes everybody hated them, but a game without challenging foes is not much of a game at all...what was bad was when they were no longer deadly, just hard to kill and swarming in packs...I think any kind of flying creature should have lower health and therefore be easier to kill in one or two blows at a higher level, and if there is a swarm of them, the others should flee when you kill one or two
The very fact that a game should be challenging is why enemies like cliffracers are not needed. Challenging is when you encounter an enemy, and you are killed, so you have to rethink your strategy, or come back at a higher level, cliffracers are not challenging, they're just annoying.
Now I have to agree that the game needs to have werewolves, or some other type of were-creature that is appropriate to wherever it takes place, preferably more than one if suitable, and you need to be able to become one, this could be like Bloodmoon where there are quests associated, or it could just be something that can happen.
Other than that, though, I found Oblivion's selection of creatures to be rather bland, goblins and minotaurs are fine, but they get boring if they're all you fight, I'd like to see a return to Morrowind's strangeness, that doesn't mean we need nix-hounds and ash ghouls, those were good, but might not fit in every location, I'd like to see some more exotic creatures where appropriate, maybe some sort of Lovecraftian horror, or even some less overused mythological creatures would be fine, the Greeks and the Norse weren't the only people to create mythology.