The first character to get the Nintendo DLC treatment has been revealed to be MewTwo. This is a character that hasn't been in the franchise since Smash Bros Melee, so I think there's a lot of excitement for MewTwo.
MewTwo will be offered for free to those who buy both the 3DS version of Smash Bros and the Wii U version of the game. Whilst he is still in development at this point in time, MewTwo should be made available at some point in Spring 2015.
Whilst he's been mentioned as a DLC character, no word has yet been given on whether MewTwo will be made available for those who do not buy both versions of the game, although the word DLC does suggest that he will be put up for sale for those not willing to splash out on both the 3DS and the Wii U version of the game.
There's been a lot of new news about the upcoming Smash Bros games recently. On top of the announcement of the free MewTwo DLC promotion, Nintendo recently announced a brand new 8-player Smash mode which will be in the franchise for the first time ever.
Whilst the game is available in some regions, the Wii U version will be available on November 21st for North America and December 5th for Europe.