Wolf if he's not already planned prior to the Ballot (But I think he's next after Lucas). Otherwise probably another returning veteran. I really don't know on Newcomers though. But I think even if it's a newcomer it will be at most a Luigified version of a character that's already in the roster (Ex: Dixie for Diddy, Tails for Sonic, Impa for Sheik, etc.). Cause so far DLC characters are a little more simple. Even with Mewtwo they just had to recreate his Melee moveset and just obviously buff it
Though like GP said they didn't say there can only be one. I feel like they could be taking suggestions so they can add characters throughout the next couple of years. For concern about Roster screen space. There was hidden files for arrows at least on the 3DS version that could leave room for a whole new page. (Not that I think DLC will add another 48-50 characters but they still can make room if need be.