The roster will have the same library of characters as the wii u version.
SOURCE:
http://kotaku.com/dont-expect-a-lot-of-third-party-characters-in-the-nex-513203533
Sakurai also stated that characters would have to be "reduced".
Theres already a thread in dismay over this interpreting what Sakurai said as confirmation that characters would be removed or the next smash would feature less characters than Brawl because the 3DS doesn't have enough memory.
I think what Sakurai meant was that they are going to change the Ice Climbers to a single Climber in the next game so that there is no lag on the 3DS version from 8 ice climbers fighting at once. I'm cool with this because it never made sense to control them both at the same time when they play solo in their actual game.
Similar reductions may happen to other characters if it could cause lag on the 3DS version, which is understandable but makes me disappointed that they have to work within 3DS boundaries on the Wii U version.