An IDE (and maybe fast RAM) interface that plugs into the A500/A1000 side expansion port would be nice. Similar functionality to kipper2k's internal CF/IDE/fast RAM board
except no need to open the case. Then you could easily connect/swap CF cards for data interchange. I think that board emulates the A600 IDE hardware, so you can just use Kickstart 37.300 or later to get auto-boot support.
A parallel port SD card interface could be an interesting challenge; see how few components you can use. Bit-bang the SPI bus over the parallel port, maybe just needing some TTL level translators??? [Performance of such a minimal interface would be pretty poor and would probably require disabling interrupts for long periods.]
But I don't think you need to design your own hardware for easy data interchange, at least with SCSI-based Amigas. Apart from using CF cards you could fit a SCSI card to your PC (lots of cheap used ones on eBay), maybe pick up another SyQuest drive so you can just swap those disks instead of having to swap the drive itself from Amiga to PC. Or buy a couple of Zip drives, one SCSI for the Amiga, the other USB for the PC.
You mentioned having DAT backup tapes. I'd recommend connecting your tape drive to a PC and imaging all the tapes before they become unreadable. One Windows program for creating an image file from a tape is Tape-IO for Windows
, there may be other/better ones. Apart from doing that for data security/preservation, WinUAE has rudimentary SCSI tape emulation which might
be enough to allow you to restore your backups on an emulated Amiga, from your tape image files.