I just read some of the ideas mentioned here, and I had some thoughts. There seem to be like three main categories of modules: Input, Output and Communications
Input can be subcategorized into Sensoric and Interface, where the first contains elements like motion sensors, gps, altimeter, compass, camera, microphone, radar detector, infrared detector, proximity sensor, radio receiver, tv-receiver, etc. and the other one all kinds of keyboards, touch screen, writing tablets, joysticks, etc.
Output also has two subcategories: Production and Interface. Output interface modules are screens, speakers, eink, braille, while production modules are something like servos, propellers, led array, beamer, hot/cold pads, printers, force feedback.
Communications be vaguely be divided into two categories: Wireless communication and hardware connectivity. Wireless is for Bluetooth, Wifi, Infrared, 3G, GSM, edge, DECT, UMTS, etc. and hardware is for USB, Firewire, ethernet, card readers and all kinds of plugs, sockets and jacks.
A fourth category would be a Core category, which is strongly related to the basic units. They aim at enhancing the device itself, with solar cells, battery packs, ram extension, hard disks, number crunchers, etc.
So what we have is something like this:
In my opinion the Sensors are the most exciting part, not only because most proposed items fit into this category, but these are also items that exist for other devices, but rarely have been accessible via a computer, because noone saw a reason to do so. Well, this community will show the world, why it’s good to have a lot of input sources.
On the other hand productive modules will be the most fun part. Because you can actually create something in the real world with this self-assembled and programmed device. This is a kids dream come to true, your own minifactory :D