As I'm contemplating the sound input and output, I deal with ADC questions.
Since I'm able to digitize with 8-bits resolution at pretty high speed, I can also digitize at lower speed, position sensors for example. Several years ago, I played with those flat "thumb joysticks" made for the Sony PSP and they fit this project nicely.
So I'm adding this new feature to the specs : 2× X-Y analog thumb controllers with 8 bits resolution. The 4 values will be available in the Special Registers space. I'm not exactly sure yet how I'll deal with the button calibration (center value) but it's not as if nobody had to solve this before.
Looking more at the PSP, its screen is pretty cool too, 480×272px with 4.3" diagonal. That's not impressive but already good for $10. It would match my 1MB SSRAM budget, with a virtual framebuffer of 1024×512 for a nice scroll margin. No double buffering, though. The CPU is not fast enough to refresh everything in 20ms...