I think it's probably gonna be Mickey or Xehanort, still possibly Riku or Aqua, because we really don't know much about either of them. We don't know Mickey's backstory, or what he was doing before and after BbS, and before KH1. As for Xehanort, we don't know MX's past, and AX's time in Radiant Garden, and who controls AX, and which part of him (MX,Terra, and Eraqus) controls Xemnas and Ansem SoD. I think there can be a series on both characters.
Aqua doesn't really have enough mystery about her to have her own series. All that's a mystery, really, is her time in the RoD, but that really wouldn't make for a good series since I doubt that there's Disney worlds in the RoD, since it's made of darkness.
Like I said before, a Riku subseries is still possible, but we already know mostly everything about him.
Edit: Wait, I think I posted this in the wrong thread -__- My computer shut off so I'm all confused. I guess it can pertain to this thread, though.
Wait are we talking about the series after KH? Or the mystery game?