With many shinigami the shikai and bankai have something in common, like with Hitsugaya, Renji, and Byakuya. With Ichigo, there's practically nothing in common between the shikai and bankai. I wonder if it somehow related to the nature of the zanpaktou. Rukia's seems to be quite similar to Hitsugaya's, and elemental one, and it's quite likely her bankai would also somehow be related to ice. Renji's is just as obvious, though not elemental, as well as Byakuya's, at least to an extent, as it seems to rely on numerous copies and such.

So, in that sense, it would be safe to assume Rukia's bankai would also be quite freezing. Well, I don't expect we will see her bankai in the series ever. Unless something surprising happens, like her becoming a vaizard to make Ichigo feel less lonely, or something...