Baseless speculation:
If ‘Everfree’ means that there’s a human-world version of the Everfree Forest, then my first guess would be some kind of spring break or school trip episode, in which the CHS gang all head off to the woods, where supernatural shenanigans occur. Maybe we’ll see some cryptids that are actually Equestrian creatures that have escaped into human-world.
Just really, really hoping that they get to go somewhere other than CHS this time.
And maybe alicorn Sunset?