User blog comment:Salubri/New Characters and Reveals in the Future of Bleach/@comment-1268838-20100828172555

Personally, I think that if there were any Shinigami powerful enough to even be considered for captaincy, that they would have already been introduced to us. It would be ludicrous to think that the Soul Society wouldn't use every single resource in its possession to counter the Arrancar threat, especially if they happened to have to at least three captain-class Shinigami that we haven't even seen yet. When the Aizen conflict is all over and done with, I think the Soul Society is going to "settle" for anybody who's even mildly qualified for the positions.

Inversely, I suppose that these fighters could have been left back in the Soul Society to hold the keep while the others were away; same for the Kidou Corps. However, that is just speculation, and I still find the idea of whipping out new characters to fill in the holes a little unlikely.