Board Thread:Questions and Answers/@comment-10797017-20130627100621/@comment-11051098-20130830144234

The 3 SWPs we know about are Shinigami. Ichigo, Kenpachi and Aizen. It could be that the remaining 2 are Shinigami too.

In that case, I would guess that Urahara and/or Mayuri could be one or both of the remaining. Both are highly intelligent, and Mayuri apparently is a master of creating poisons and stuff. Who knows what he can create.