I designed a character that suffered from a curse that left him with a kind of multiple peraonality disorder. He had four different personas and a class to go with each. His physical stats stayed the same but his mental stats would change.
His personality changes were dictated by stress, kind of like the Hulk. So anytime anything crazy happened or when combat started, he'd make a roll to determine the personality that took over. If he succeeded by a certain amount, he picked who would take over. If not, it was determined randomly.
Only played him for one adventure, but he was a lot of fun.