I also do not see the point in limiting fast RAM, just go for 2GB and be done with it.
In an ideal world then yes that would be awesome but things are not always that simple. The OS has some issues accessing more than 512 MB and the hardware becomes more complicated to build (cost and time to develop go up). You also have to think about what applications would need that much, really there's not much.
As long as a new Accelerator had say 256 or 512 MB RAM then it should be enough.
The only other thing I'd consider adding is an HD controller (SATA would be ideal).
If you start adding graphics chips and USB etc the complexity goes up for the hardware and the development of the drivers to actually use these extras.
But the biggest issue I have with sticking an off-the-shelf FPGA computer inside a Classic case is that it will turn my A1200 into a retro keyboard!
If you simple want to 'play' around with an modern mini-computer then you can just buy a Raspberry Pi instead. You could even stick it in an old A1200 case if that's your thing
I think the biggest market for an FPGA accelerator is in something
cheap and configurable (can emulate different hardware 030/68000/060 all with or without FPU)...something very similar to the Vampire with more power and memory.
And then make it available for all Classic Amigas