I've always liked the idea of having two different mobility modes. I loved the robots in 2000 that could move along the central bar, and I think that sort of thing is a lot of fun to build and watch. It would be neat to have a couple of ramps, maybe a foot high, separated by about 4 feet of space, with a bar about five feet above them (imagine the 2000 field, but cut widthwise across the ramp, move the two halves apart, and rotate the overhead bar 90 degrees). That way, teams could choose to either traverse the bar to get across the gap or somehow climb up and over a one-foot vertical wall. That's probably complex enough, but if you really want to get ambitious you could have a monkey-bar type lattice of metal pipe that robots could climb on.
As for the field surface, I think carpet is pretty good...plywood lends itself to becoming dusty and slippery, and I shudder to think what sand and dirt could do if it got inside the motors, gears, sensors, and speed controllers (and it inevitably will get inside). If FIRST used a surface that allowed more traction, there would probably be a lot more teams that would stall and blow their motors.
-Ian Mackenzie
Woburn Robotics
http://www.team188.com