When DLC was first announced for the game, every few months or so, new characters in addition to stages were added. Nintendo even held an official character ballot were you could vote which characters you wanted as DLC. One of the more popular votes on the internet was Shrek, though many felt that was stupid, with only a handful who were not only positive about it but also knew about his own fighting game. I would like to see him in a future installment, though I doubt that is going to happen. On a related note, there's also these.