The obvious idea is to have a single FPGA board with RAM, large enough to hold the full AGA/OCS/ECS/RTG/CPU cores, and have a I/O connector.
Exactly. And it's not just a great idea because I mentioned something similar in another thread...

The more I think about it, if you could design such a board with different daughterboards for the various systems (1000/500/2000/3000/4000/standard PC case?), not only would it be the same main board for all of them, but it makes future upgrades easier..
With FPGAs, the minimig is maxed and you can't just upgrade the FPGA, you need to design a new board.
Same thing with the FPGA Arcade. It's just about maxed, so any "major" speed increases will need a new board with a newer FPGA.
(Not counting expensive daughterboards with actual 060 chips on them.. ;-)
Fine, if you have the same basic connector, when a larger FPGA is affordable, a new run of main boards for those who want them...
Next year, I buy the model with AGA and 50Mhz 68020 version and a 1200 daughterboard for my 1200.
But in a few years, there's a new model, AGA++ with 68060 at 200Mhz, so I upgrade the main board for my 1200.
But I also get a 1000 daughterboard so I can put my older one in the 1000... etc...
And, like the FPGA Arcade, there's no reason to limit it to the Amiga.
Hmmm.....
That might be worth paying more for....
desiv