So far I heard a desire for 3D vector graphics (like in 80s/90s scifi; should be negligibly any harder 2D; from @phryk), audio filtering, making the abstractions optional, & an interpreted language like BASIC. (@millihertz)
I see mixed opinions (@vertigo) over whether CPU should generate the raw wire signal, or if that should be buffered in RAM. As a non-masochist not nostalgic for C64... I have an opinion...
Is it feasible to have a full MBs-large framebuffer with consistent timing?