Rukia was portrayed as a princess dancing on the frozen lake, so I expect that her Bankai will portray that even more, thus her Bankai might be a dome-type (like Tousen's, Byakuya's 2nd Stage) where a gigantic frozen lake will be created, which she will use to her advantage, for attacking, defending and alike purposes.
I'm a bit surprised that no one was even alluding to this:
but, don't worry, I have already prepared near perfect video when Kenpachi enters the stage, kukukuku (now, where was it ;P ?).
On another note, it's also possible that the gang will be portrayed as the 4 Heavenly Beasts, you know, Tiger - Renji, Dragon - Rukia, Phoenix - Ichigo, Turtle - Byakuya.