KeeperFX really allows you to fiddle a lot with the game and I've been toying with it for a while, mostly creatures for now.
Imps get more spells; teleport is moved to lvl5, they get invisibility, heal (which sadly they refuse to use for now) and fireball at higher levels. Training imps is actually pretty appealing now, even outside of the sandbox maps. They also slowly gain exp while working.
Orcs are faster and slightly stronger. They need a lot more time to reach lvl10 now, at which they get rebound, turning them into mage-killers. They also make a very good counter to Mistresses and Vampires.
Casters in general receive a ranged attack at level 1. Warlocks have a better spell lineup, at the cost of longer training at higher levels. Ghosts, while still not particularly powerful, are at least viable now outside of gimmick plays.
Flame breath is reworked. It can stun enemies for imprisonment and it deals more damage, but you can't rapid-fire it in first person, making it stronger in hands of creatures, but weaker during possession. Dragons also get a melee attack and are now deadly at close range. They also get a fireball at level 3, so they aren't completely defenseless at range until meteor kicks in.
Mistress is a bit squishier and gets lightning later, but receives speed and drain sooner. Vampires also are a bit less tanky, slower, get teleport later, don't receive armor and have reworked training curve. The also start respawning at level 5.
Demon Spawns have more health and receive fireball and flame breath at highest levels, turning into mini-dragons.
Tentacles get hailstorm at lvl10. They also attack fairies and priestesses in the lair.
Barbarians hit harder, while dwarves are tankier. Monks have slightly more health than other casters, giants got a massive health upgrade, archers are faster, beefier and get sight. Knights are all-round stronger and will fuck your shit up. Priestess is immune to gas now.