The mole miner could be a fun enemy. All the others... meh...

By the way, +1 to the Magnetron. Definitely an element that adds difficulty to gameplay, but not with direct damage.
Anyway, was about to make a thread like this too, but I hope you don't mind if I merge it here...

I just figured that we primarily needed more Beasts in the game, so I had a few ideas:

Gruboom
Basically a grub with an oversized, pulsating abdomen filled with an explosive substance. Upon defeating it, you'd better pick it up and throw it fast, or it might just explode in your face for a colossal amount of damage. Grubbooms explode instantly when thrown, like Fire Jars, and can damage nearby Knights.
- Rationale: Improvised explosive device.

Stunky
Black and white-striped skunks that look suspiciously like Wolvers, and dart around the battlefield fast, suspiciously like Wolvers. Unlike Wolvers, however, their attacks are manifested in the form of a caustic liquid cone spray that widens as you go down the tiers. At Tier 3, Stunkies can spray liquid in a ring.
- Rationale: Fast-moving ranged attacker, can inflict poison AND other statuses.

Scaler
Corpulent, seemingly adorable dragons that pack a nasty punch by firing elemental bullets at the player. Oddly enough, fire Scalers are only weak to fire, ice Scalers are only susceptible to freeze, etc. And after receiving their respective statuses, they get enraged, taking more damage from players, but turning their seemingly harmless bullets into massive elemental balls that pummel into the player for high damage.
- Rationale: Low defense, high damage, encourages players to take a risk in attacking it.

Nimbaas
Fluffy as a cloud - oh wait, its fleece is made of clouds. Lightweight and bouncy, cheerful almost all the time, Nimbaas enjoy prancing about the Aurora Isles. That is, until you attack them. Enraged Nimbaas release a nova of elemental energy periodically until defeated.
- Rationale: Useful for area cancelling, and cute. Hurr hurr.