For the record, any suggestion of a MLP RP has usually been laughed and scoffed at with half-hearted "yea sure, we'll play as brightly-coloured, talking ponies with shit on their butts..." Does it happen? No.
I feel like it would end up like that awkward Lion King RP. I'm just sayin'.
truth
Also, the new MLP (particularly MLP:FiM) sucks and they're disturbing looking. If you were going with old-school 80's MLP the movie/cartoon RP instead, I wouldn't be so disturbed by this thought.
That's a little unfair though. The original cartoon is dumb, childish, senseless, asinine and a relic of a bygone era. G3 ponies are even more disturbing than the FIM series. Infact any version of the ponies is worse than FIM.
At least FIM is watchable. I'm not a 'brony' as the term goes. I wasn't that taken with it, but really, it's pretty good. It's got good storylines, it's not overly childish, beyond what I'd expect, and the designs are really good. As is the writing. It's genuinely funny. But then I suppose we have to thank the creator for that, given she also made Powerpuff Girls, which as we all know was pretty hilarious, AND fun to watch.
Also my only objection to an MLP:FIM roleplay is the point I stated before.
Maid. Princess. Celestia.
I guarantee you there are THAT kind of fan out there, they WILL want to play, and before you know it the only RP that's going on is Rainbow Dash and Pinkiepie having vaguely sadomasochistic non anthro pony lesbian sex, and Celestia will be "sexy ms fanservice" for everyone else. OR there will be six Celestias all with god powers, and equally thousands of Luna's.
Any originality will be swamped with Mary Sues and fanwanking.
That's the only reason this is a bad idea.
Which is, by the way, the same reason any OTHER RP of games or animes or cartoons or films is generally a bad idea. It ALWAYS devolves into this.