I'm with zferolie on this one. The remaining top picks first party characters who are not already fighters have all been seen as spirits, AT and/or costume now: Waluigi, Isaac, BWD, Elma, Rex, Dixie, several Zelda contenders, Paper Mario, Chibi Robo, Springman, Ray MK III, Rythm Heaven characters...
Like I said on other thread, the remaining possibilities are small. It's either:
Really obscure characters. But that won't sell well, so it won't be that.
- Not represented franchises and future games characters. That can be the case for some like Dragon Quest, Minecraft or Phoenix Wright, or even the future Fire Emblem or Pokémon generation, but I really can't see all of the 5 DLC being that.
- Being a spirit means nothing and, if we want to go further, we may even think that being an AT neither. Some will get upgraded as fighters.
I believe it will be a mix of the 2nd and 3rd option. Hopefully, we will have several waves of DLC.