Skeever, mudcrabs, etc.: level 1/2 to 2
Wolves and so on: level 1 to 4
Bears: level 10 to 20
Bandits: level 1 to 20(bosses)
Guards: Level 5 to 20
Mages/Necromancers: level 5 to 40
Vampires: Level 10 to 40
Giants: Level 25 to 30
Dragons: Level 25 to 50


etc. etc.

This is a very rough idea with rough numbers, but it was what I was hoping the scaling would be like. Obviously, damage and durability should be considered - a necromancer should do lots of magic damage at high level, but that shouldn't also mean they get the health of a bear. Currently, many things have simply inappropriate stats for what they are. A bandit should never be more durable than a dragon when they're wearing hide armor. NPCs that have obvious reasons for being very powerful should scale pretty high however. Also keep in mind that just because something caps at a certain level, doesn't mean it will completely stop providing challenge - NPCs can come in groups. A pair of the strongest types could still be a challenge for a level 50 character.