a possible idea for the STS2 world: Deadly Livestock!
Exploding Chickens: exchange the data values of creepers with chickens, so that the AI behaviors and mechanisms are switched (Creepers are harmless, chickens chase you and possibly explode)
Cows... more like Bulls!: Change out the AI's of Cows for Zombies
Squid can bite, too...: Change out the AI's for Squid and Skeletons, so they chase players and shoot arrows in the water
I Dream of Sheep: Sheep and Endermen are switched, so if you stare at a sheep it will flip out, port and start attacking you like an enderman.
Spider-Pig!: Spiders and Pig mechanics are switched
Silver-Wolves: Wolves follow Silverfish-AI, possibly change the #92 silverfish egg data value from looking like stone block to grass block?
Hell-Bats: Bats now shoot charges like Ghasts, making them the one and only threat to exploring Caves (if normally Aggressive Mobs are changed to become Passive)
Firin' Mah LazerCat!: Ocelots follow Blaze mechanics and may shoot blaze balls.