Occasionally in fiction you'll see a character who controls when they switch powers (or temporarily gain powers), but has no apparent control over what power or powers they get. The classic example from DC comics is
Robby Reed, and other users of the
H-Dial, but I'm sure I've seen some non-Gadget-based versions as well - one possible example is Schmendrick the Magician, from
The Last Unicorn (based on
the novel of the same name), who seems to work best when he lets the magic do as it will; this may be something more like the
Whimsy spell from S. John Ross, though.
The simplest way to cover most of these options, IMHO, is Cosmic Modular Abilities with a 'Randomize' limitation, though I'm really not sure how much of a limitation it should be (the old
Whimsy as Powers thread had suggestions, but didn't come to a conclusion, and was pretty specific to the Whimsy spell). My current guess is below, but suggestions would be welcome:
'
Randomize, -40%
You control when your Modular Abilities activate (switch abilities, or
grant abilities if the power is normally dormant), and what the abilities do when you get them, but
what abilities you actually get will be determined by the GM, standing in for chance, the gods, or some other entity. If you pull your abilities from a limited list, the dice may usually make that decision, instead. If you also don't control what the abilities do when activated (like poor Schmendrick, and other Whimsy spell users), that's probably a specific variant on the Uncontrollable limitation, and if you don't control this power at all, add Unconscious Only.'
I vaguely recall a mod saying that if you aren't sure how big a limitation should be, try -40%, so that's what it is at the moment. Not sure how well that works, though.
Thoughts?