It might be happening. Eventually.
Tonight, during the "Cissy" episode of South Park, a pony was spotted that looks like Twilight Sparkle or Cheerilee. That Eagle-eyed TV-fan Spazz spotted it during this scene.
The plot of tonight's episode covers topics that have been prevalent in the MLP fandom for quite some time, including gender roles, and basically tumblr. It is also another episode that has directly referenced an ongoing storyline. Is there a pony episode in our future? Would that be a good thing?
If one did happen, who would be the "brony" character? Cartman does already own a Spike plushie...