The existing speedometer tells you how fast you're going in any direction, but it would be really handy to have a sensor that senses motion in only a given direction. A speedometer version would be simpler to use, but I think having it work as an accelerometer might be the more interesting/challenging option.
For a straightforward usage example, you could attach one pointing backwards to activate a set of reserve engines or a dodge mechanism when your sumobot senses it's moving backwards, or one pointing to the side to sense when you're drifting sideways.
On the more complicated side, this would allow for some interesting vector math in combination with the existing absolute speedometer and the directional sensor, setting up triggers based on angular velocities and such.
