Another issue is the problem of quantity and quality of the powers available. The Twilight Sparkles, Zettas, and what have you are on a tier of demigodhood that most normal players just can't even hope to contend with. Even Chie, with her own sense of powers, takes more of a defensive supportive role and let the big guns do their things.

I would like to second the encouragement, then, of drawing on the resources and abilities of other players as a source of finding appropriate solutions. A part of MLP that I always found appealing is that no one particular race is more superior than the others (unless you count the snobs up in Canterlot). The Earth Ponies have their tie in with nature, the pegasi can fly and change the weather, and the unicorns can use their magic in powerful and meaningful ways. Challenges can be presented therefore in a way that utilizes these talents -- a food shortage or a situation where resources are scarce is a good example. In this case, earth ponies may help doing work in town, pegasi will try to do what they can to make sure the weather works right, and the unicorns will help find creative ways to use the resources that they have.

I would be careful not to pigeon-hole players into being expected to do what they are, but it's certainly a way to draw in more creativity than ubbers beating up ubbers.