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"Wouldn't this be a unique thing to do with this character? I mean, think about it: Sunburst thought that he would be a powerful unicorn after he went to Canterlot and studied more magic. When he got there, however, he learned that he was not as powerful as he thought. Then he moved to the Crystal Empire and after reuniting with Starlight and seeing that she turned out better than him, how do you think he would feel? Sure, I bet he was happy to see her, but the knowledge that he is not that great a wizard kept eating at the back of his mind.

I doubt that this will ever happen, but it would make sense. He turns to dark magic, possibly even become a student of Sombra (if he's still alive) and learn that the friendship he shares with Starlight is more powerful than anything else (it would be rather similar to the lesson Discord learned at the end of Season 4).

I don't know, though. Truth be told, I just wanted to draw an evil Sunburst with Starlight watching in horror as he stains his mind with dark magic. This would add an interesting twist to her story, though, wouldn't it?"