Evidently it's been possible to run NEXTSTEP 3.3 on an FPGA SPARCstation 5 for a year or so:
There is also a 486 core (and various 680x0 cores as part of Amiga and Mac implementations)—I wonder how good the compatibility is? :)
MiSTer FPGA systems (https://www.retrorgb.com/mister.html) are basically the hardware equivalent of MAME; something like 150 cores have been written and run on their hardware specs. (The weird capitalization is inherited from the MiST project, which was an FPGA emulation of, among other things, the Atari ST.)
MiSTers seem to typically run anywhere from around $150 USD for a naked Mini-ITX board to €400 for a prebuilt system, making it roughly in the same ballpark as a prebuilt Raspberry Pi, but with the added satisfaction of an FPGA implementation instead of Linux running a software emulator.
Does anyone have any experience with these, or an interest in getting one? I can imagine a future where someone CADs up a Mini-ITX-friendly NeXTstation case to house such a board. :)
I have one of my misters running this core (from temlib (https://temlib.org/pub/mister/SS/)). It saves me quite a bit desk real estate.
The link also has tarballs of various OS's that you can use with the core.