2) We can, but then it won't be able to do much else. My basic idea is 4 '51s: CPU, Audio, Video and I/O. All of them will be loaded nearly 100% when performing their respective functions.
4) Again, the question whether we use different derivatives for different components, or all the same. If so, a single USB-enabled chip i.e. for IO would be nice, and I DO mean slave chip - i.e. for syncing with PC, network connectivity etc. This is completely optional though.
5) With separate chip we could make several options. The basic idea is your bulk storage is a PC (over RS232/USB) and your "current" storage is some built-in eeprom/flash/NVRAM. The "advanced" idea is to give it ability to use harddrives and floppies, and whatever you desire. The USB storage requires USB master and quite a bit of software and hardware wizardry. For now, not really an option. Boot device would be up to the user.
6) 2 joystick ports: Absolutely yes! :D Optionally one/both mouse-enabled :) And since it would be rather hard to get any other kind of keyboard at reasonable price attachable with reasonable easiness, PS/2 is the way to go. A PC-style parport and 2 serials if we aren't short on pins with the IO chip (IDE swallows quite a few, and we still need a common address and data buses!