I think she will obviously gain immortality, flight and lots of powers that make her a princess. She might have a chunk of land to rule over and it would be nice if, since twilight comes between night and day, that her mane would become a mix of Tia's and Luna's, like the outside of her mane being a warm, gentle orangey-yellow color while the outsite retained that indigo starry mane that is much like the night sky. Her power will either be the most powerful magic in equestria or it could be friendship. I don't know if I would want her to outlive her friends though.