Why not use a Parallax Propeller 2? 160MHz @ 0.5IPC go zoom.
Well OK I can think of a lot of reasons why, just thought it was funny this was mentioned in the FAQ at all!
Just for the sake of it, and for completeness, I'll list why I think it'd be a bad idea:
While the P2 could actually handle keeping up with the external bus (It has interrupts unlike the original, with 4 cycles of latency), you've just added a processor that's many, many, many times faster than the main processor and at that point you should just build a P2 computer.
And the good: P2 can do up to 1080p VGA and also handle high quality audio with it's own DACs, which would mean a chip count reduction. Could even repurpose some of it's memory to reduce chip count further by getting rid of some dedicated SRAM.
Anyone know why the Propeller question is even in the FAQ? I was quite amused to see it there.