Twilight created new magic and understood the magic of friendship.
But yeah, Fluttershy probably deserves alicornhood too.
But, I think you also need to be able to perform an elemental magic. (Something in nature, Friendship, Love, etc.) You don't have to just understand Friendship, you just need to understand some elemental form of magic. (Cadence didn't create new magic, she just understood, on a deep and profound level, the magic of Love and how it could help ponies.)
I dunno, but if any of the others were also on the path to alicornhood, it'd probably be Fluttershy. (Her magic probably being something in nature.)