Power/weapon swappage. *wants to see Lenalee or someone with a huge claw for a left arm* Haha, wonder if anyone has X-ray visionnnn~
Or or, speech modification? Like some characters can only talk backwards, in a foreign language, to certain people (so like, a chain where one person can only talk to one other person, and that person can talk to a different person), or not at all...?
All the ideas you put sound really funnn. Especially gender swapping. xD