Next Spring, Mewtwo will be available for free to players who own both the 3DS and Wii U versions of Super Smash Bros. or and will be up as paid DLC at a later date.
A representative at the Kyoto corporation told Gamespot this:
“Mewtwo will be made available to purchase as add-on content in the future, and is also available as part of the Super Smash Bros. Club Nintendo promotion, which offers consumers different ways to obtain this content. There are no other plans regarding paid add-on content at this stage.”
Director Masahiro Sakurai turned down the idea of bringing more DLC to the table because it would involve a lot of money as well as a good number of people. Mewtwo may possibly be (at least for now) the only DLC fans will be able to obtain.