ALL captains have bankai, EXCEPT Kenpachi. I remembered this in a much earlier chapter. I mean if Hitsu has one, Soi Fon obviously would have one too... I can only imagine Bankai's getting much bigger, because when Byakuya saw Ichigo's bankai, he was like: "That puny thing can't be a bankai". However Soi Fon is part of the "assassination/ninja squad", so like her shikai, it would probably be small and deadly. I would think it would be similar to Halibel's resurrected form, giving her more speed, defense, and abilities (e.g. projectiles).