For peripherals I'm thinking a trackpad you can draw on, keyboard (transparent upon eink display, so you can programmatically change keycaps), 3-axis joystick, & a drawing tablet. As well ofcourse as a monitor & speakers!
I'm considering allowing peripherals to load sandboxed drivers into the coprocessors to parse its data for the main CPU to process. Though bandwidth concerns...
As for building an OS for this system... Even a minimal one could get tricky...
3/4!