From commenter on Derpibooru (spoilers):

Team all but confirmed the return of Discord (not just DeLancie).

DeLancie has lots of unused lines from previously that will probably be in the DVD bonus features.

Meghan used to use the term pegacorn, but after seeing what fans were using, all the writers now use alicorn. Jayson says the fact that alicorns have only been seen in positions of power is definitely by design.

I’m pretty sure Jayson and Meghan were just speculating for this one (so watch fans label it confirmed) but Jayson said he’d really like to visit the griffon kingdom in an upcoming ep, and Meghan said she’d like to see “Luna’s abandoned moon colony.”

the new villain is called King Somber, Discord reappears, star swirl was mentioned, they're adding a new mythological creature.
also this: Jayson says they try to keep pony technology limited, with even the filmstrip projector chosen because it’s not modern. Meghan joked we’d never see Twilight tweeting. Jayson later had to shrug his shoulders about the night-vision goggles seen in the clip.