are we just talking graphics, or sound as well? More channels would be nice, as well as the option to use 16 bit samples, and to set the volume independently on left and right speakers. Perhaps the upper byte of the volume register could be used to set the volume for the opposite side? We could maybe use a bank switching scheme to select banks of four channels each, like AGA uses to select palette register banks.
As for the blitter... well one thing that would be nice is if you could use the C source during line drawing, as well as to use the Bresenham (line drawing) circuitry for other things such as image scaling.