Ok so this headcanon + Twilight's design is heavily inspired by, but (hopefully) not completely stolen aha ^^' Some aspects were inspired by both their looks in the climax of the friendship games (I consider EG canon)I imagine that although Celestia and Luna won't die of old age, they will eventually officially leave their thrones. So Twilight and Sunset will rise to power as the new main leaders of Equestria. This would include them each taking over raising the sun and moon- Twilight would raise the moon and stars, and Sunset the sun. Celestia and Luna will still reside in the castle, and both would be considered Dowager Queens. In this pic, Neither Twilight nor Sunset are at their full power yet (constantly flowing hair, etc). Twilight is more powerful than Sunset at the moment as she has been an alicorn much longer, and Sunset still needs to readjust to Equestria and its magic.I imagine they're talking about one of their friends-Starlight Glimmer, but Idk yet aha ^^