A proper tested and verified on as many as possible cards as possible: SCSI<->SATA bridge. What is out there today is scary and messy. If my motherboards were alive I'd really want an SSD... Or just a new scsi controller that really is a SATA controller.
A proper in-place programmable and externally reset-able 1M ROM ($F00000-$FFFFFF). Possibly as a Z2 programming device, or a clockport thing, or serial? Or something memory mapped. Needs many models as all Amigas are pretty different.
And if doing 3000/4000 cpu accellerator: How about trying to put some sram on the cpu card, short cable it to the chipram-slots, and fiddle about the access so that all cpu cycles gets local speed instead of crossing the chipset bus? (Yes, you need some arbitrator for when the chips wants access.)
