i suppose in theory, other devices could be added. years ago i heard of a network adapter for macs that plugged onto the scsi port. but then, god knows how they did this.
even if you were able to get software to use the scsi controller as a bus manager, what kind of glue logic would you need to bolt on GFX cards and the like? and who would write the drivers?
nice idea tho. instead of an external scsi box for desktop A1200s, you have an external card cage? - expanding on this idea, i have seen PCI extenders that use a fair length of ribbon cable, so might it not be more realistic to have some sort of mediator that instead of having pci slots, threw a couple of ribbon lines out to an external card cage where you could bolt in pci cards?
or how about a mediator with no PCI slots, but had an onboard NIC, GFX (or maybe mini pci?), sound, USB etc, in one compact board for desktop machines, and had a breakout box with all the ports on it?
nice idea for the scsi route tho. all of a sudden A500s, A2000s, et al. with scsi can have PCI expension too! :-D