my gateway into transformation was physical transformation affecting the subject's mental state/instincts/ability to think clearly and that's still kind of my favorite type! i like when the tf adds or takes away a sensory system, i like focus on the feeling & internal experience of having the brain replaced with inanimate material (plush stuffing, pooltoy air, robot conversion). i like having whoever/whatever initiated the transformation present to tease and be handsy
i prefer gradual transformation over poof-and-you're-done; i'm less interested (but not uninterested) if the subject starts human and ends human, although twinning specifically contains some fun possibilities

