The FPGA Replay has 32 MB DRAM which should be enough for many applications. For ethernet the board space needed is PHY+Jack w magnetics so it's easier to have it on base board.
FPGA Replay has USB which is wired to the ARM according to the
schematic page 2, but if it's available for AmigaOS is another matter
There is only one experimental add-on board with MC68060 socket and Ethernet asfaik. And it's not stackable. But there shouldn't be anything that prevent such design.
The video toaster flyer would need an external video A/D + D/A. Dunno how fast the original 3x disk interfaces was but maybe the SD-card can do. Otherwise an Spartan-6 rocketport can connect one S-ATA disc per two differential ports. Processing power is another matter.
Storage over IDE / SCSI is easily implemented in the FPGA without any physical incarnation.
