The finale brought up quite a lot of questions. First of all, are Celestia, Luna and Cadance born alicorn, or were they tranformed? If it is the latter, how did the first alicorn appear, and who did it?
Another thing about the alicorn tranformation:
Twilight now has SIX small stars in her CM instead of five.
What exactly was it that turned Twilight into an alicorn? It looked like Celestia was the one turned Twilight into an alicorn, but I think the reason is the spell. Star Swirl's failed spell alters destinies by switching CMs. When Twilight was finishing the spell, she was also reading it. As the result, the spell was activated, her CM changed and so did her destiny. The ethereal realm might be the place where the alicorn ascension takes place, and Celestia was just there to initiate the ceremony. I still can't think of why were the elements involved in the process.
And what does the 6th star represent, Twilight's "completion"? It probably doesn't mean a new main character. I really hope not.