Yep, Sunset Shimmer turned out to be a pretty great character. I know a lot of people would be upset if this happened, but I would love it if she showed up in season 5
It's amusing to note that Sunset Shimmer's great character comes almost entirely from EG2. She was nothing special in EG1 (actually, most of the ponies in EG1&2 are pale imitations of their FIM originals), but EG2 took EG1's simple "defeated villain" and turned her into something interesting.
Sunset in FIM S5 would be okay, but Sunset currently owns the EG series, so she should at least finish things up in EG3 before coming over, and that would make FIM S5 "too soon" for Sunset's arrival. Maybe season 6? Ideally, she should have a worthy exit inside the EG series (unless the EG series is cancelled before she can make her exit), but she should also have an introduction/entrance in FIM once she arrives there. Each series should show a different side of the door, and the crossing should make sense from both sides. I do think Sunset should move back to FIM sooner or later though.
Serious question: Human Sunset Shimmer, or Pony Sunset Shimmer?
In the past, Hasbro has dropped human girls in with the ponies, to make them relatable, and Lauren Faust apparently said "That ain't happening on my watch." But Lauren's watch is long past, and Sunset Shimmer isn't exactly human, she's a human-form pony. They could potentially use Sunset as a nod to G1, without falling into the trappings of G1, if they're smart about it. Unlike Twilight, Sunset has grown to enjoy things like walking on two legs and having fingers, so it would make sense for her character to
want to keep her human form (or at least occasional access to it). Plus there's gag potential with Lyra (and EG2 proved with the stealth Bonbon shipping that they're well aware of the fan characterizations). Also, I would say that Sunset actually looks better as a human, unlike the rest of the EG cast.
Bringing Sunset Shimmer over to FIM as a human would be a risky move, with plenty of shark-jumping potential, so obviously it would depend on whether the writers were up to the challenge. Since I'm assuming the writers
are up for the challenge, I'd say the risky move would be a good idea.