Twilicorn was terrible and defeated the purpose of the show, which was for Twilight to learn the magic of friendship. There was no reason to make her a princess or alicorn at all and it only served to imply that Twilight would outlive her friends.
The castle was very unnecessary as well, because what was wrong with the Treebrary? Why destroy such a structure we had loved and cherished many memories inside of in favor of reinforcing the whole princess thing?
Glimmer was handled so poorly I'm amazed they actually lacked enough braincells to make her equally/more powerful than Twilight, the literal element of magic. Her redemption should have been more gradual as a character arc instead of a heel-face turn.
Don't get me wrong, I'd love to keep glimmer as more of a "different aspect of magic" sort of thing, but as she was in the show, she wasn't written well.