Seriously, a water game wouldn't work- here in California there's a drought.
I would personally love to see another game with lots of robot interaction and alliance strategy. Like Aim High, but maybe with an extra element or two to the game...
Another random thought occurred to me: since the robots will use Wi-Fi to communicate with the Driver's Station this year, what will happen when the people in the audience inevitably try to create about 20 different ad-hoc networks? I know there's 11 channels for 802.11, but they can overlap slightly in terms of interference, so there might be communication issues if things fall the wrong way...