For Rider, I always wanted to play a version of Medusa where her skills would allow her to turn from a Rider into a Berserker if you forced her to keep using her more beastly nature. Like tapping into 'Monstrous Strength' too many times for too long and letting her restraint slip away and then... bam! After some Mental Pollution and Mad Enchantment, Gorgon Medusa~ Just an idea though 

(It'd be a heck ton of skill though that would pile on her though, [s]unless made into one skill that would describe the transition... That gave skills after each checkpoint... yeah no way around it[/s])