Yes just yes Discord is Starswirl the Bearded!
Well I have no proof of this. It is just my favorite headcanon, that I really want to be true.
My theory is that Starswirl was celestia student A long time ago, much like twilight he was supposed to ascend into a alicorn, and help princess celestia rule over Equestria. But the problem was that to be able to become an alicorn, a pony is supposed to have complete mastery and understanding of there talent. Starswirl the Bearded talent was magic and the most powerful magic in MLP is the magic of friendship, but Starswirl was never very good at making friends and didn't find friendship too important, So he was never able to unlock his true abilities and become an alicorn.
The spell in magical mystery cure was supposed to turn a pony into a alicorn but Starswirl could never figure it out and when he used it on himself it turned him into Discord. Celestia and Luna turned Discord into stone hoping that one day they can restore him, but they never could. So that is why they asked twilight And friends to reform him.
Plus the interactions between twilight and Discord becomes so much more fun and interesting when you think Discord use to be Starswirl. Especially now when Discord is trying to teach Twilight lessons and things.
So ya no proof i just think it would be cool.