I like the trainers idea, the various generations have some really fun outfits that could easily be color-coordinated with the various ponies.
Something else I'd been pondering doing myself was doing a series of "My Little Indies" making pony version art of various popular Indie games, such as Bastion, Limbo, Meatboy, etc. You can look at game genres like Platformers or RPGs. Could also do something crazy like pony Derby Girls. Ah dunno, just bouncing ideas around.