I don't actually anticipate it to ever happen, but if it does, I think the first one to get confirmed will be Isabelle, like @
praline
said, considering she is everywhere now.
After that, there needs to be a new Zelda character. From what I have seen from the old polls from before release, the requests for all of the Zelda characters outweighed some of the biggest numbers by a lot, and Zelda has gotten a lot of growing. Now, the only non-Assist Trophy choices are Vaati, who, while cool, is really obscure, Tetra, who isn't outstandingly unique and another form of Zelda, and Impa, a side character who only recently was even considered by the fanbase. If Sakurai seriously wants a new Zelda character for DLC, I don't think he would say "well, we've spent all the most unique and popular Zelda characters, guess we just have these guys".