I grew up on a lot of punk rock and casually find myself playing the air drums when listening to my favourite metal bands. How cool would it be if the air drums actually made sounds? What if you could play drums without actually having a drum-set?
You'll need accelerometers to detect jerking motions (your hands beating on invisible drums). These accelerometers could communicate via bluetooth, wifi, or even a custom channel to a receiver that sends it into your computer using USB. The other device is a simple receiver that does any conversions and feeds it into the computer & DAW:
Detecting the position of the hands and feet.
There a couple of ways you could do this. One way would be to set up positional sensors on your wrists, elbows, and shoulders to measure the pitch, yaw, roll and translation of your arms. This problem is well known and studied heavily in robotics. When applied to robots it's called the Six Degrees of Freedom. 6DoF. The other option is to use a Triangulation system to detect the position of your hands and feet in the x-y-z plane. This method offers much less sensors attached to your body.
The device would be MIDI controller so that integrating it into DAW like LOGIC, Fruity Loops, or Ableton would be a breeze.
Educating the kit.
So how would you set up a kit? The idea is that you would train it. By hitting a button in your DAW it could go into record mode. Then you flick your wrist (air drum) at some point in space. The accelerometer attached to your hand registers the x, y and z coordinates and sends them wirelessly to the device. The device listening for these coordinates sends them as a response to the DAW, which registers that point in space as a drum. You are then able to map that registered point to a sound inside your DAW - whether it's a tom, snare, crash, ride, cowbell, what-have-you.
After you've downloaded a template kit, or trained your own - you'd put the device into play mode (as opposed to record mode) and try it out. When you flick your wrist in the vicinity of one of your registered coordinates, the DAW would play your instrument.
This 'vicinity' should also be configurable in the DAW. If your drums are all very close together, then, having a high vicinity value may cause some drums to bleed into others. Having a too low vicinity value may make it hard to 'find' your drums and cymbals.
Consumer Goods. Sell at music stores across the world. Or, open up shop and sell online only.
Delays. Everything mentioned above has to be done in as-real-time as possible. I've played guitar with as small as 30 ms latency and it is very noticeable and makes playing hard because you're brain get's out-of-sync.