Exercise isn't fun for a lot of people, but everyone should do it (particularly those who spend a lot of time playing computer games). So why not make exercise somewhat more bearable to do by combining exercise with video games? I'd like to see an exercise bike turned into a PC / xbox360 compatible game controller so you could play racing games with it. Steering would be via the handlebars, and analogue acceleration would be based on pedaling speed. It would also be good to have a few extra buttons somewhere on the handlebars to handle any other inputs needed by the games you played.
The more compatible the controller the better (imagine playing a platformer with a pushbike - crazy!)
I'd like to see it able to be quickly and easily attached and removed from an existing bike so you could use it on days it was too hot/cold to go out on your real bike.