Weird. I'm running an M&M campaign set in a fairly open setting, and pretty much every player came straight to me with a good reason for having what they had.
The best one was one I worked out with a player - his character, a runaway apprentice mage, managed to rediscover a very limited form of shapeshifting. I wasn't keen on the variable shapeshifting or metamorph mechanics, so we ended up homebrewing an Alternate Form setup.
Because she's an apprentice mage AND she's still learning the ropes of that school, she doesn't have anywhere near the capability to use the ability without straining herself severely, so shapeshifting requires a Fort save every time she shifts.
That's anything *but* flavorless.