[quote=@Abstract Proxy]
Is something like temp shifting plausible (polymorph/shapechange)?
[/quote]

Very much so.

Using an example of a demon-ish character:

If someone's foundation of logic for their magic is "the world is nothing more than a sandbox that I can smash around," their magic would function along the lines of something outwardly destructive. Throwing fireballs, breaking structures, etc.

If their foundation is more of "they may be right, I am a demon," they'd have the associated power of a demon (there are many types), but they'd also be able to safely manifest physical attributes like black sclera, horns, etc (the person above can also technically do the same, but it's irreversible and a consequence of duress).

If their foundation is more "My self is destruction and my body is merely a tool," they'd be able to have a wide breadth of changes they can freely control.

You don't have to think of it as phrases as I have. That's just the deep analysis of it. For most characters, it'll just be their unconscious thoughts. If you just want a power, you don't need to deeply justify it. There's like a 99% chance you can just have it so long as it's not permanent/resembles immortality.