The best thing to do is come up with a new ability or two. I, unfortunately, did not do that, so my cards were somewhat limited. However you should make at least the Mane 6 or something, see how you like it. It's amazing how many variations exist of the Mane 6, and rightly so.
I'm actually doing the opposite first. Trying to find the most obscure clips from the show that only a fan would recognize and make abilities for them, my biggest issue is the fonts. The cards don't look right with the default CS5 fonts I have. I know there is 3-4 different fonts that WOTC use on the cards and there hard to find it seems.