It's not just unlikely, this is nigh impossible. I'd love to see them too and I think they're perfect for Smash Bros, but the fact is that they're now second-party characters to a competitor. If it was possible, Banjo and Conker would have certainly been in Diddy Kong Racing on DS.
Furthermore, Sakurai has
suggested that we might not even see any new third-party characters, as he'd like to keep the focus centered on Nintendo and its history.
If we do see a third-party addition, I'm almost certain it will be Pac-man anyway on account of the game being co-developed by Namco. He's already appeared in Mario Kart in the arcades, so his chances are pretty good. While I'm not a particularly big fan of him as a character, he was essentially Mr. Videogames until Mario came along and he'd be a hugely significant addition.