"But Princess what about my friends?"
"Twilight you are a princess now, your duty is no longer just to your friends but all of Equestria."
So I was right about Discord converting, well it's time for another one of my theories, ahem...
So I think King Sombra will return and rein terror and hatred of all of Equestria, but Lun Celestia and the coverted Discord will try and stop him to no avail, the only way for them to win is for Twilight, the inbetween of day and night; chaos and harmony, to become an Alicorn. She will defeat Sombra and Celestia and Luna will welcome her as a princess but Twilight has to choose between ultimate power and friendship and if the title of the show was any hint she'll choose friendship and give up her Alicorn powers to stay in Ponyville with her friends. I don't know it's just a theory it might not happen but just as with the Discord conversion theory I might be right >;D