Well this is a great vector. Now there isn't much I can say about this. Wait there is.
It seems you took the concept of the "dark" magic from the Crystal Empire and applied a more pure, "light" version of it to help reinforce power or at least make Twilight seem more magical. Now what really stood out to me were the wings. The wings being not a part of the actual body really work for me. With them being a construction of energy, it adds more to the overall work. I liked the use of glowing eyes and the horn glow ties the other traits together.
Your vector appears to be very show accurate. The proportions are done well. The only things that could improve I can say would be the mane and hair. It appears to be a little too rigid but that may have been done as part of whole power effect.
Anyway, a great piece and keep up the good work.