I wonder what you will use for video generation in the C256, because the greatness in 8-bit gaming came from these chips. So you'ld have to differentiate there or run the danger of becoming yet another retro knock-off (well, the merits of compatibility should not be underestimated. Most of the effort will be in software otherwise). For the Gigatron it was ok to have limited video effects, because its insanely low chip count was its primary objective: so we have no tiling, no hardware sprites and no color indirection. But we do have indirection per scanline and you can do neat things with that, such as the bending road in the Racer game.
One thought is to design your own video circuit and put it in an FPGA. ASIC is also an option if you go with a 0.18um/0.25um foundry (X-FAB). You can also do it with discrete logic, but it will become big. There is the Vulcan-74 guy doing TTL (well, CMOS) video circuits: http://forum.6502.org/viewtopic.php?f=4&t=3329 and http://sleepingelephant.com/ipw-web/bulletin/bb/viewtopic.ph... (video: https://youtu.be/EIghk1BUG98)
Another idea is to do a hardware version of the PICO-8 platform: https://www.lexaloffle.com/pico-8.php
For sound you should really consider the awesome SAA1099 chip.