As a total X-Men geek, I can confirm for you that Legion is actually not that new. His first appearance was in the mid-80s. He's been written with a variety of mental disorders besides dissociative identity disorder. What he has is completely dependent on the writer who is handling the story, from schizophrenia, dissociative identity disorder, to a form of autism, and a mix of all the aforementioned. Which probably showcases the serious problem of writers writing about mental disorders and developmental disorder that they don't fully understand.
Legion is certainly one of the more interesting characters in the series. He was born from the most powerful telepath on Earth, so one could only expect him to have extreme powers of his own.
Let's hope these writers will handle Legion and his disorders with the respect and realism people who suffer from them deserve.
"Don't be ashamed to need help. Like a soldier storming a wall, you have a mission to accomplish. And if you've been wounded and you need a comrade to pull you up? So what?" Meditations of Marcus Aurelius, Book 7:7