For now it is impossible that she come back. Why Sunset Shimmer is now happy in the human world, and with the exception of Twilight, has no friends at Equestria. Everyone who knows and loves are in the human world. In the future maybe she can do it, but not now. At best, he could decide to return after graduation. Because, after taking it, she should leave the school. And leaving the school, what she can do of herself? I admit that even after graduation she may decide not to return. And if she will not return on that occasion, she will never return. If she come back after graduation would be nice, because it would show that the adventure and experience of Sunset Shimmer in the human world is over. And in this way we could see her make peace with Princess Celestia, and in general we could see what kind of princess might become (because surely she will not merely sit around the cutie map along with the mane 6, but would become a princess different from Twilight).