The more I think about it, I believe physics and polygon collisions should be next if possible at the same time. However, I know Ashley and his group are probably firing all cylinders to get stuff out, so whatever gets done would be great at this time.
If I had to choose, it would be physics as it can completely change the mood and dynamic of a game.
In full disclosure, I originally chose multi-player, but that was before I got my PubNub sample working.