I'm going to say this now before your idea gets shot down further down the road, but the fact that your OC is an alicorn, "the chosen one" a former human who dies and wakes up in Equestria, AND marries Twilight smacks of Gary Stu-ism. Actually, and I don't meant to sound harsh here, but this is probably one of the most blatant Gary Stus I've ever seen. I STRONGLY suggest you stop a moment and sit down to revise him.