Hmm, I think terms from other games would be cool. Some ideas:
Level 7 : Magikarp
Level 25 : Mudkip, Munchlax
Level (doesn't matter) : Snorlax, any other Pokemon
(And since the level 70 rank is Legendary, these would fit) Level 70 : Entei, Mewtwo, Groudon, Rayquaza, other legendary Pokemon etc.
Level (doesn't matter) : Bokoblin, Poe, Octorok, other Legend of Zelda monsters etc.
Level 30 : Metroid
Level (doesn't matter) : Goomba, Bob-omb, Thwomp, Chain Chomp, Blooper, etc.
Well, you get the idea. Adding classes of creatures in other games appeals to the fans of those other games, and it may encourage them to try and get those classes and become more active on PMC. Hope you add these!