[BlackNhite]

Since we're on Aokiji, I'd bet his weakness would have something to do with salt.
I know salt "melts" ice or something to that effect and it also composes half the devil fruit weakness. The only problem with that would be I find it a bit too mundane for Oda's tastes to use the same weakness twice like that.

Then again, maybe Luffy will never have the chance to defeat Aokiji. I'd love to see what would happen if he were to go up against Ace; considering he escapes it could potentially happen. Hell, with the recent surge of "team-orientation" Oda's taken such a liking too, maybe we'll have a little brother team up at some point.