Short version is: there's an abundance of people that want this or have ideas on how it could/should be done, and a shortage of people doing something about it. It doesn't help that every time someone does do something, someone else helpfully says you should have used an FPGA and then doesn't offer to actually help make it happen.

Long version: I've made a super rudimentary hand-soldered 030 PDS card that only has a declrom, and wrote a valid stub of a declrom for it. It's too much of a rats nest to really do anything more with it. My next step would be creating a PCB based on the hand soldered board with some breakouts for experimenting with attaching other devices. But I'm not willing to do PCB layout. I started and it's not for me. tt did a bunch of work on this for which I am eternally grateful, but there's still some routing to be done. Anyone working from my rudimentary design will quickly find that more complex address decoding logic is necessary, which means more chips or cpld or fpga, and all the associated development and layout issues.

Nubus is significantly more involved than PDS, and no one seems to be able to pull off a PDS card.

Another option: I've written a Serial Disk device driver, which you can use in conjunction with the ROM disk to get more (slow) storage. Not perfect but the advantage over most other options is it actually exists. There's some talk of a standalone serial to SD card device by techknight or landonf.

I've made some efforts trying to put the serial disk driver into the IIsi ROM, but it hasn't gone that well. Maybe someday.

Summary: It's entirely possible to do and there's plenty of documentation available. If you're interested, go for it.