My favorite theme is forced feminization. Surgical, chemistry, maybe hypno. At the end the victim has to look pretty hot. Not realistic but it gets me going!
I like the stories where the MC has agency and embraces the changes/transformation (or at least admits before too long that the intial resistance was an act) more than the stories where it is forced on them against their will. I guess I'm a bit vanilla like that...
I like the ones where the changes are slow and gradual and the protagonist is largely unaware of them/self-rationalizes them. E.g GF secretly dosing the guy with hormones