What trait in a subject do you find enjoyable to transform?

I’m a big fan of more procedural/temporal/reality changes, I think. That probably sounds weird, so let me explain.

You might be familiar with procedural programming. It was a big deal a few years ago, when a computer game called Spore came out, made by the same guy who makes the Sims. The game itself was shit, but it had a revolutionary character/creature editor. Basically, you could design a creature with however many arms, legs, etc. you wanted, and the game would figure out, based on what the player did, how to make the creature move in a sane, understandable fashion. It does this (as far as I understand it, so don’t flame me if I’m super wrong, because I’m just making a metaphor here) the programming has a system of broad scope rules regarding how limbs behave, and when the player gives it a unique set of limbs, it adapts those broad rules to the new instance, and (hopefully) it all works out like it’s supposed to! 

See, you can always change a feature of a character–that’s easy. Make them fat, make them short. Make them a twink. But make them follow a new set of rules, and that’s where the fun begins. Like, make a character unable to bathe in anything other than piss. Or, make them compulsively consume 3000 calories of food every time they get a hard on. These force them to change, but it also a) gives the character space to react and try and manage the change as best they can, and b) can deliver surprising and unexpected results, that simple changes can’t deliver. 

I fit temporal and reality shifts into this broader category as well, because they also have unexpected and wild side effects. The butterfly effect, essentially.