Like it or not, the possibilities of female additions in the next Smash's fighting roster are minimal. Nontheless, we must keep hoping for slightly more even female equality in the cast.
That said, the following is who I think would have the most chances of being playable in the next game(or the one after that, depending on the case);
Krystal(pretty much a like it or hate it character, though she did get mentioned in Sakurai's journal 3 times)
Dixie Kong(she was a sidekick in DKC 2 AND became a hero in DKC3. Any questions?)
Mona(considering Wario's recent "Wario Ware" trend, she seems to be the most logical representative of that series after Wario himself)
Lip(she got mentioned in Sakurai's journal twice, so who knows?)
Micaiah(One of the main roles in FE:RD, the series' most recent entry)
Capt. Syrup(if Wario Land: Shake it! sells well, she might have a chance)