You know what I would like to see invented? Automatically-engaging stabilising wheels for bicycles. Bikes are fine when you are bowling along at some speed, but you lose the gyroscopic stability as you slow down. Also, when you are riding in traffic, you are continually slowing down, putting your feet down, and engaging your feet on the pedals again--all of which is inefficient. On a recumbent bike, the speed at which instability strikes is lower, and getting the bike started is harder. Tricycles are great in this respect; you can stay with your feet clipped in to the pedals for maximum pedalling efficiency, and there's no problem about stability at low speed. But trikes are big and heavy so you use more energy just shifting the extra weight, and the extra wheel and frame sections make them expensive.
In my opinion, the ideal compromise would be a pair of stabilising wheels, which would automatically lower when the speed dropped below a certain level, then raise again as you got back up to speed. Ideally, the speed threshold would be adjustable for different bikes and riders. I have a very simple computer on my bike which measures my speed quite accurately, so the technology already exists for that part. It would also need a small and light motor to raise and lower the wheels, but as they would be lightweight and fairly small it could be a done by a small, battery powered motor.
The result would be relaxed, efficient commuting, and increased safety in the event of emergency braking, and the wheels would even form a handy stand when parking the bike. The main sticking point would be how good it looked, though--no-one would want to be seen dead on a bike with training wheels, so they would have to be designed with a hi-tech, sporty look. If anyone feels like building a device like this, I would happily be a beta tester!