I don't really care about the 2600 core. I already have a dozen ways to play 2600 games on a modern TV. Intellivision, ADAM are top priorities and I will be stoked if the Intellivision sees a stable core on this.
My wish list would be the Astrocade, Spectrum, AMF MP 1000 Dream Machine, TI 99, Apple IIe, other early computers as those do not have a lot of ways to play around using the technology of the Phoenix. C64 is not needed as there are a lot of ways to play C64 games and emulate the computer in a way that is close to the original.
Also nice would by an Odyssey2 interface.
5200 would be a waste of time given the analog nature of the system. NES and beyond have a lot of support already from other projects.