The series finale. Give the mane 6 what they all set out for in season 1, have them really prove they are the spirits of harmony, and finally have Twilight replace Celestia, just as faust had intended. Have a morally grey villain,like a Starswirl that returns from the future, and attempts to be rid of the mane 6 because they end up causing a huge cataclysm due to their link with the human world. Have all the minor protagonists return (including gilda, trixie, sunset shimmer in pony form) and really just have it wrap up the FiM series.