So, I have the theory that in Rainbow Rocks will be focused in Rainbow Dash and this time it's her turn to cross the mirror and see for herself what happens in the human world, which also makes me guess that there will be a total of 6 EG movies, each one focused on each pony who will have their turn to cross the mirror.
I mean, IMO I don't think Twlight may have reasons or even would want to go to human world again, and after she told what happened I'm pretty sure the rest of the ponies would feel curiosity or at least a reason to cross it will be given.