Heroes of New Game+
I discussed this a little in a recent blog, but it's been an established trope that a new game+ mode of several games will allow you to play as a "guest" protagonist. Usually, this version of the game is a little tougher, because the character has a markedly different style, lacks many of the benefits of the original protagonist or is simply a joke character designed to get their ass whupped as a genial way for the developers to troll fans who felt the game was too easy.
I've tried not to include: DLC characters, bonus characters from ensemble games like most RPGs and hidden characters in Fighters (because I'll be here all week..). I'm sure I'm missing plenty of obvious candidates, regardless.