I might be jumping the gun (figuratively), but I can't wait to see the background ponies receive their time in the spotlight. Even if it'll only be for a single episode.
From what I've read, some of them will have personalities akin to the fandom's perception. Others will be different. I've no problems with it, and hope the fandom doesn't flip its lid... again.
In any case, I'm almost current the following will be true (but please don't quote me on it): Octavia will be prim and classy. Vinyl will be a rocking DJ. And they will be besties.
Bon Bon might have a new voice, and somepony (like Lyra) will point it out. Speaking of the minty green unicorn, she will more-or-less be the Pinkie Pie of the group. She's hyper, emotional, has a major sweet tooth (if her reaction to her pie being eaten is any indication), and might even break the fourth wall.
I hope Minuette, Time Turner, Derpy, and a few other fan faves appear as well.
Regardless, the 100th episode of MLP will definitely be something to be remembered.