Even though Zack was...incapacitated at the end of DOAX, it's still possible that that whole game was merely his fantasy, and he will be there in DOA4. But with the addition of Lisa as his new partner, that means another character will be required to give every character a tag partner (Hitomi was his partner in Dead or Alive 3). If this is the case, what will the new character be like? Man or woman? Fighting style? Connection to the storyline?
Another option is the rumor that Ayane will die, creating a spot for Hitomi to tag with Hayate. Now, Tecmo has never removed a non-boss character completely from any previous Dead or Alive game, but the story of the Japanese characters seems to finally be getting some substance to it, and anything could be possible. Personally I'd be really pissed off if they kill my favorite character.