Sunset Shimmer is a fantastic character. I think takai right back in Reply100 sums up my thoughts on her pretty succinctly. I love the way Twilight Sparkle is developed through the series, yet Sunset Shimmer does almost the whole lot within a single film, and from a worse point! It also helps her design is adorable!
So conspiracy time!
I think Sunset Shimmer may make the Mane7! So reasons for this:
It is a common trope for ensemble casts to gain new main players, although we have the cutie mark crusaders and the royalty, the main ensemble has not increased. The story has examined many of their personalities and traits, adding a new player can shake things up a lot!
The new character is usually a total badass and/or a reformed villian who beat the ensemble in the first place (check, check annnnd check)
The addition of said character is often mid-season or after a canonical film (as films are more willing to shake the status-quo whereas episodes are not), though they may have been introduced or fore-shadowed many episodes before. Season 5 is on hiatus till after the film, maybe there is a storyline reason for this as well!
Sunset Shimmer is not part of the human world, and EG3 trailers indicates that human Twilight has a major role to play, if Twilight joins and makes the HuMane6, there might be realisation for Sunset that the human world is best for the humans. (also human Sunset might turn up)
The Mane6 are not in balance anymore, they consist of 1 Alicorn, 1 Unicorn, 2 Earth, 2 Pegasi. They need another unicorn to restore harmony! The 3 unicorns I could see filling this role are the reformed villians: The Great and Powerful Trixie!, Starlight Glimmer and Sunset Shimmer. Sunset, however has had the most character development and is already friends with Twilight.
Sunset Shimmer has become Twilight's pupil, much like she was Celestia's. This is a great storyline to develop, showing the growth and maturity of Twilight Sparkle now she is the Princess of Friendship. Since the EG films are less frequent, the best way to do this arc is have Sunset Shimmer move back to Equestria.
And finally Sunset Shimmer is best pony!
So my favourite Sunset Shimmer Ship!
My favourite pony Sunset Shimmer picture thus far
My favourite Human Sunset Shimmer Picture thus far