That I know of, there's the stuff with the kids, the skinned Predators, the alternate ending and probably at least a couple of other scenes mentioned in the scripts, characters from which are still listed in the film's credits, implying they were filmed and considered for inclusion at some point. In their commentary, the Strauses also talk about filming some Predator POV shots that never made the movie. And I'd assume the original final act where Dallas' kid brother dies exists in some form, as that was supposedly changed after a test screening.
Nothing groundbreaking, but out of curiosity I'd be interested to see whatever they have. As it is, the film's easily the shortest in the franchise, and I remember reading somewhere that it was cut quite considerably for time shortly before release. (Because Fox realised they had a stinker on their hands and just wanted to maximise how many times it could be shown in a day?)