Ouch, that would actually be painful (not in a bad way).
I don't really have any ideas on what would happen, but I can take a few guesses.
- CMC get their cutie marks (at least 1 of them)
- The rest of the main 6 become princesses (Although this would not go down too well with me, nor would it make a lot of sense)* - Possibly the final season of MLP:FIM, as it would at least end on a good note (Well, we can only hope it will be good)
- New villain in the first and last episode(s)
- Rainbow Dash gets in to the Wonderbolts team
* I don't think the main 6 all becoming princesses would work, for a couple of reasons
1) Turning earth ponies & Pegasi into Alicorns would make little sense, and strip them of their natural appearance
2) There is already 4 princesses, adding another 5 would be a HUGE jump and make no sense.
I don't think many of my predictions will be true (but it would be cool), but we'll have to wait and see