The following is very much a rumor, and should be taken with a grain of salt:
-According to rumor, Fuwa Juuzou's human name seems to have been "Shiba Juuzou". Juuzou was the younger brother of Takeru's father, Entei.
-Juuzou, jealous of his brother for becoming ShinkenRed, may have betrayed Entei to Dokouko, allowing him to eventually kill Entei. In return, Juuzou was granted access to the Sanzu River, while he was still alive.
-It is rumored that Juuzou will reform to fight by his nephew's side as "ShinkenWhite", whose kanji will be the character "十" (Ten- the number, as in Juuzou).
-ShinkenWhite's weapon is a giant Jutte/Jitte:
http://en.wikipedia.org/wiki/Jutte
If the above is true, the ShinkenWhite drawing we saw could be any of three things:
1). A fake (which I don't think it was)
2). An early rendering of ShinkenWhite (this is the most likely)
3). Some sort of fusion of ShinkenRed and ShinkenWhite, perhaps in a Burai gives his powers to Geki kind of way