What The Future Might Bring

or Glimmer, Glimmer, Sparkle, Shimmer

Personal headcannon for where the show could eventually go...I mean alicorns aren't immortal are they? They live a long time, but everyone has to die one day. So Twilight, Starlight and Sunset could become the next alicorn trio - like Celestia, Luna and Cadence are.
Or Celestia sends the three of them off to rule a land that's very much like Equestria was at the very start. It would be a cool thing to see.