I would guess it would be any letter of the alphabet except A, B, C, D, E, or F, seeing as we already know people who have those names exist.
A and B (gyuuki jinchuriki and current raikage).
C and D (the guy who got one-shotted by Sasuke's genjutsu at the summit and one can assume logically Darui would be D because him and C were there together).
E and F (we know there is an F, as a White Zetsu was disguised as him and B recognized him, and judging by the patterns we're seeing, I think all letter names have a partner, and therefore there must be a E, even though we haven't seen him yet.)