Thinking about forms you can “transform” into by way of being “put” into them. Maybe your actual form doesn’t change (or at least it doesn’t need to) but it’s still effectively a transformation because it’s a significant state change and can’t be undone so easily by normal means. For example, getting coated in slime that acts like an extension of you, but your unchanged form still exists in the center, maybe even visibly… or being put in some kind of outfit or costume that you can’t take off, maybe because the gloves are too clumsy, or because it’s cursed in some way. Really cool robot armor that syncs up with you a little too well… that sorta thing.
Of course, over time, it might transform the body within to fit it even better, especially if it’s cursed. The idea is that it’s a “form” that exists because of things that aren’t necessarily parts of your body.