This sounds fun - I like that it is an extension of the CPC hardware, not something different.
So this replaces the CRTC and Gate Array right?, and when installed those originals are 'disabled' somehow, so they don't take the bus, so the new hardware can? Or is everything duplicated in the extension RAM so both run at the same time.
Is there much free space in the FPGA? Could you add a simple blitter or some sprites or copper or similar - given time of course? Dual-playfields, heh :/
With the CPU + DMA card, you're a lot of the way to a faster standalone CPC replacement. Thoughts about FPGA Z80 instead of physical?