If we're going strictly by Tsukihime, all I can think of is... Arihiko? Makihisa? Roa/SHIKI/etc?
But we already have Kouma/Len/etc who aren't Tsukihime characters, so... Akira? Nanako? Zelretch? Crimson Moon? Hanei? Souka? Princess mode Arcueid? Some of them don't really see to be reasonably capable of fighting others (Arcueid, Nero, etc.), but we have HisuKoha and the like in the game already, so... If they limit themselves to Tsukihime/Kagetsu Tohya characters (as they seem to have done so far, other than joke/original characters) there aren't that many possible choices left.