I second sea ponies! Plus, we haven't had a "beach episode" yet.
I'd love it if any
G1 pony showed up, period. Complete with the right colors, or at least some other visual cue to clue fans in, other than just the name. In fact, I was incredibly disappointed to see that none of the background ponies, save maybe (MAYBE) Bon Bon and a green Medley-like pegasus, were references to the rest of the franchise. Guess Transformers spoiled me in that regard.
But speaking of likelier inclusions... I'd say Minty and Toola Roola.
I'd love to see where they'd take Minty's G3 persona, which, judging this series, would be awesome
(friendly + naturally nervous + extreme OCD + clumsy + cranked up to eleven = BOOM).
And Toola Roola, well... I can totally picture her as either an artist looking for inspiration or an aspiring designer... Who'd probably look up to Rarity. ...Come to think of it, doesn't Core 7 Toola Roola have most of G3 Rarity's colors? References!