I’d like to see a special/movie that’s the inverse of all others to come before: Someone (human) stumbling upon the portal and going through to wreak havoc in Equestria, with the help of technology and/or magic. Bonus points if it’s Sunset’s human counterpart, but at this point I’m not sure if she even exists. Starlight’s human counterpart would also work, because she could (somehow) lock away the real Starlight and impersonate her for a while.
Given that it’s supposedly really bad (according to Celestia) for too many to cross over from one side to the other, I’d suspect it’d be only Sunset, Human Twilight and Dog!Spike who would go over to help. That way we could have Twilight and Spike awkwardly getting used to being other species, Sunset loving being back in Equestria and using magic regularly, and being faced with a hard decision to stay or go back. Of course, she’d go back but not before meeting with Celestia who’d make it clear that she’s welcome any time.
Yes, it’d basically be a multi-part FiM episode disguised as an EG movie, but it’s about the only way we could have “Sunset in FiM” (sort of) and finally end her arc that began with running away (that she’s still not faced up to).