Called it.

Been talking about it for weeks now, no one believed me. But who's laughing now? HA! Alicorn!Twilight.

further theories of my own (let's see which one will be true):
Crackpot prediction: Spell goes wrong, Celestia and Luna are knocked out of commission, Twilight panics and strings spells together, the combination of them causing her to become the Alicorn of the stars. She fixes everything. (This being the first 8-12min of the episode)
Twiluna ensues and the world-building from "Sharing the Night" becomes canon. (this being the remainder of the episode).

... one can dream, no? But I suppose a more realistic prediction would be: "they mess the episode up, multiple characters will be OOC and hasbro executive meddling ensues" - not necessarily in that order.

edit: I just stumbled across a way that would arguably be even worse: "twilicorn happens, twilight stays an alicorn, no one gives a shit.", that'd pretty much kill alicorns, mess heavily with the fan interpretations of luna and celestia (... and one could make the point that it'd contradict previously established canon) and well, probably eliminate any chance of a luna or celestia episode. Or a tleast one that isn't godaweful like mare-do-well or the spike episodes were.

other theories I liked:
TheLastBrunnenG

Super-secret story arc spoiler: Celestia arranged the magical cutie-mark mishap in the first place! She's playing chessmaster, seeing what Twilight would do if given power. This was all a test. When she sees that Twilight wants harmony and justice more than she wants power and glory for herself, that's when she knows that Twilight is really ready for coronation. The Ascension will take place - but in secret. Twilight will hide her true power and her wings except in emergencies, hauling them out only when all Equestria is at stake.

WandererD

Now, I don't know about you... but I'm waiting for Celestia to confess that she's Twilight's real mother. via a drunk party with Discord and Dr. Whooves