Looks like I got here just in time!
I have been avoiding the forum because it moves so quickly, I can never get involved in the conversation. I will try to pop in from time to time though. Sorry.
Also, I put something together that you might find interesting. It is a set of stages that every newly discovered background pony goes through in the fandom:
Stage 1: Discovery, marking the character out as unusual or interesting.
Stage 2: Naming, usually follows shortly after stage one.
Stage 3: Fan art, often accompanied by anthropomorphic art.
Stage 4: Complex fan works, including animations, Tumblrs, fanfiction, and anything that requires a great deal of time and/or commitment.
Stage 5: Relationshipping, can include romantic pairings, friendship, enmity, or any combination, the main point being that the character’s interaction with the official cast.
Stage 6: Fandom consensus, which occurs when the fandom comes to a general agreement on the character’s traits, personality, and behaviours. Other characterizations and/or names will either be forgotten, merged together, or explained away.
Stage 7: Official recognition, when the creators and/or Hasbro recognize the character’s existence, this step may or may not involve an “official” appearance in the show.
Stage 8: External recognition, occurring when the character is alluded to outside of either the show or the fandom – hitting "mainstream" popularity.
Examples: Octavia, Scratch, Lyra, and Bon-Bon are all hovering between 5 and 6 (Lyra got thrown out of 6 during the "Heartstrings" debacle, but is working her way back now). Derpy is the best example of an 8 that I can think of (Even Penny-Arcade has alluded to her), while Dinky is about a 4 as far as I can tell. The new pony Allie has also recently hit 4, as someone has made a Tumblr for her.
There may be some overlap between some of the steps, and a character may not follow all of the steps in that order, and likely will not complete the whole list, but I think that this fits in with the general trajectory that a recognized background character will follow.