The FPGAARCADE is your only hope now that the Natami fire has fizzled out... but that is only going to support a GFX card mode, and not a superset of AGA.
The Natami is still in development, just without any major public announcements lately. The mainboard is currenctly getting a redesign. I run a beta system at home with an earlier board revision.
I agree that the FPGA Arcade seems like a well designed system though. I will probably get one when the expansion board is available too.
Regarding the original question, a SAGA card for Amiga would require piggybacking just about every custom chip and would probably be super unstable due to all the connections
You are better off picking some RTG solution.
