A nice Commodore 64 almost compatible with extensions. Better graphics modes with 128kB of video memory, 512kB of paged system memory, 40kB of main memory, a good emulator, 8 channel FM sound and an FPGA graphics solution.
Good 6502 development tools are available and the ROMs are open source. You can buy nice USB keyboards to help support the project. The 8-bit guy does many YouTube videos on classic microcomputer technology. He has good support of the retro community and if it were not for a design constraint of using as much off the shelf older parts then it would be a single chip thing. But part of it is the hands on experience, as the emulator on a Raspberry Pi would work.
The physical hardware is still under construction. As an 8-bit system without an accelerator it is suitable for new code or conversions of old. The main goal is to prevent scrap by the off shelf requirements and make an easier to code for machine than the Pi. One person can understand it all given time.
I’m doing a factor analysis of the hardware docs now to check how I’d use it. I think a scan limited 512 by 448 graphics 4 bits per pixel mode is one I could use for games and other software development. I’m sure a nice palate of colour can be made for the index and offset constraints of the graphics.
It has 16 hardware sprites and more of the system is becoming finalized. The sound was recently tested and works after sorting out some 8MHz timing issues. Some IO is still in design but it is looking good.