I like the idea of ISA cards.
I don't know if it's possible, but the VGA/EGA/CGA/Herc video card would be exceptionally cool from my point of view... especially if you could manage Voodoo-compatible acceleration. I know the ISA bus is very slow, but still, it would be cool, imo, to be able to install a voodoo-compatible card in an ISA-only motherboard and play *some* 3d accelerated games and apps.
One nice aspect of such a card is the idea of flipping a switch or running a utility and switching from CGA/EGA/VGA/Herc. It would help software development so I wouldn't have to swap out cards or set up a separate system to test new code on. If it had multihead capability, like that one old Matrox card, so you could run a two-display system, that would be useful, as well.
I also very much like the idea of the GPIO breakout. For awhile I've been wanting some form of GPIO board for ISA PCs. Main idea I have is connecting to LEDs, switches and displays on a custom-or-customized case.
Also such a board might be usable with homebrew, custom-built CNC machinery.
Another idea, which probably isn't possible, is an FPGA-based accelerator card. Operate your PC normally, then flip a switch and reboot and it's running much, much faster. Like maybe turning an 8088 into a 486 or Pentium. With, of course, the limitations of the ISA bus. One application would be to ensure compatibility with older software. You can cruise along on a "486", then when you get an old piece of software or game, you can switch back to "native" mode.
I know some people may criticize this idea with "might as well run an emulator"... well, one of my fantasies is to make my daily driver computer a 486 or a Pentium running dos, and maybe multibooting into some old flavor of Linux or BSD. And not run on modern hardware that I increasingly distrust.
Another possible card, maybe, is a dedicated encryption card. So it would handle SSL handshakes, allowing you to use legacy software/hardware on modern sites. One of my other fantasies for years has been modifying Arachne to somehow handle SSL so I could visit modern websites. My most recent ideas involve running a proxy on a modern Linux box. Not only could an ISA encryption card provide SSL for web access, but ssh for shelling onto Linux and other systems from older PCs.
Going at it from the other end, an ISA and/or PCI adapter for the Raspberry Pi, so you could plug ISA/PCI cards into it. Why would anyone want to do this? I dunno. But it sounds neat to me.
Also, a Raspberry Pi-on-an-ISA card adapter that would take either a PI, or a Pi Zero, or a PI compute module, and plug it directly into your PC's ISA slots.
Again, not sure of the uses for this, but sounds neat. Or maybe simply an arduino-on-ISA card. One thing they would do is maybe give you access to the variety of sensors and controllers available for the PI/arduino.
I've seen PCI-e ISA adapter cards, and I really liked the idea except pricing. Basically a backplane you can plug ISA cards into, that connects to a PCI-e slot.
oh! One card I would like is an MPU-401 card. Definitely. Even as part of a sound card.