I've been thinking about how this would work, and one idea I came up with, is that casting magic should work as you said, but if we're going for a magical girl/henshin theme, those are traditionally pretty theatric and devestating, so one way of doing it is invoking a "small" thing carries no consequences, but doing something "big" is not only a more difficult roll, but causes mental (and possibly physical) stress damage so that they become appropriate as the "finishing move" in these types of things.
You're in the driver's seat at this point, but what do you think?
As for the "picked by a Goddess" thing, it might be cool, but the abstract goal of "Love and Justice" has the potential to be restrictive while still nebulous. Also might be cool to fight "evil" magical girls.