Wi fi is possible, but people use it only as a proof of concept, it will cause a lot of headache to work and require specific wifi chip.
As far as I remember, someone asked few days ago whether the 360 usb joypads were compatible, and no one said they were not, since they don't need special drivers under windows or mac it is possible they will be usable with the BOB, but if I were you I would get myself a cheap usb joypad that has enough buttons for SNES/MD/GBA etc... and spare the 360 joypad for 360 gaming.
beside the game pads (which means mutli player in PicoDrive and MAME), you can attach a dance mat for Beat2x, a hard drive (you will suffer though cause BOB only support usb 1.1), a keyboard, mouse, Card reader for common flash cards, and... that's what I have in mind right now, have I forgetten anything guys?