I recently played "Zero Tolerance" on the Mega Drive, one of I think 2 real FPSes that ran on it. It's an impressive feat, but even then the viewport was about 30 lines high and not even full width: You could probably squeeze a little more performance out of it with modern development tools, but a "ROM Only" port of Doom would look closer to the VIC-20 version with more colours. Using a Mega Drive console as an interface between an FPGA, a CRT, and controllers is an interesting concept though...
See the "Doom on NES" posts earlier in this thread. I think I mentioned a similar "SNES on NES" project too. It's all been done before.
