The Why of It:
As I said in my first message, I'm not hardware knowledgeable. I didn't intend to phrase my post as "I'm going to do this" or "somebody should do this". I was asserting what I wanted to do and what I wanted to learn was whether or not it was feasible.
I'll tell you why, in no special order, I would like to do this. There are probably better alternatives. I don't know them, and I won't discover them unless I stick my neck out and open my mouth about it (figuratively speaking).
A) I hate the way they've been turning computers into dumb internet terminals, appliances over which I have less and less control. Everything's "out there" in the "cloud". And one way I'm prevented from turning back the clock and hanging on to my *computer* as a computer *accessing* the internet, is with calculation intensive things like SSL encryption. My phone spies on me, my computer spies on me, my browser spies on me, but more importantly, I go ballistic when I'm told by my Windows 7 box that I'm denied access to a file or function because I need administrator privileges... even though I *am* the administrator, with supposedly full administrator privileges.
My phone is a sandbox I don't really control. I even tried doing a little Android development, wrote a couple things... and it made me miss the days on my Amiga when *I* designed the user interface to suit my program, not to match every other program on the machine.
B) I went the Commodore route; Vic20, C64, Amiga. It was when the Amiga died that I turned to the PC, and so I missed out on an era. I'm making up for it now in some ways with DOSBox and GoG, but I missed out on tinkering with and programming for x86 PCs, for the most part. One of my sincere goals is to "devolve" into using DOS 6.22 / WFW 3.11 / DesqView based machines for most of my day-to-day tasks. Oh, sure I've got a modern PC running Windows 7. It's not the latest and greatest, but it'll run the Unreal Engine 4 development environment. But for word processing, web surfing, scheduling, light graphics work and entertainment, I want to, as much as possible, do so under the ancient OSes, preferably on ancient hardware. I *also* want to develop games based on those old environments, using those old environments. (So the argument that nothing would ever use a Pi zero emulating a Voodoo card doesn't matter, because one project would be for my games/apps to include support for it).
C) People have built, and are selling, AdLib parallel port devices. In some cases, you have to go in with a hex editor and edit the binary of a game in order to get it to support the device. Yet it was still built and people still want it. *I* want it. People have designed and built XTIDE, an IDE drive controller on an 8 bit card to allow even XT class PCs to access more modern drives, including CF card drives. Perhaps my suggestion of a Raspberry Pi or Arduino might have been overkill for my goals, in which case I'm guilty of ignorance. I still think a 3D accelerator card, emulating a Voodoo card, just ISA, *would* have an audience.
D) As I also said, I've been wanting to get a 486 to use for my main computer. I'd like to be able to stick a 3D accelerator in it, not only to be able to play Doom and other games that support 3D acceleration, but also to try to *write* them. And I'll need appropriate hardware to test as well as develop on. (8-Bit Guy on YouTube just finished writing a new game for the C64, right down to packaging and selling it, and has begun work on a DOS version... but much of the work he did for the C64 version he did on a C64 or via emulation followed by testing on real hardware. He's inspired me to give DOS game development a try).
I currently have my old K6-2 300mhz desktop with 256mb ram, a Voodoo2 3000 (iirc) PCI video card with 16mb ram, and an AWE32 sound card in it. Definitely overkill. I intend eventually getting an XT and/or 286 class machine, as well (I'm now considering building the XT from a kit I came across online). Combining C & D, there are, and always have been, 8-bit VGA cards (I've got a Vega Video 7 still in box with disk and documentation). So if I were to write something to support 3D acceleration, and it supported a custom, 8-bit 3D accelerator, you could see the absurdity of an IBM 5150 possibly playing 3D accelerated games.
E) As I said, I just got a Toshiba T5200 laptop. It's a behemoth, but it has 2 things I wanted very much; an orange gas-plasma VGA display which I don't believe you can get anymore, and two ISA expansion slots; one 16 bit, one 8 bit. I hope to be able to oneday stick a Cyrix 486DLC accelerator in to replace its 386DX-20. In the 16 bit slot I've already replaced the Artisoft lan card with a 3com Etherlink III (sadly lacking a boot rom socket). I had hoped to place a Sound Blaster 2.0 card in the 8-bit slot (yes, the card is PC/XT, 8-bit, not AT 16-bit). But the card is about an inch too long. Now I'm going to have to get either a parallel port or an ISA remake of an Adlib card in kit form. which is why my interest in hardware hacking has been piqued. I actually toyed with the idea of gutting the T5200 and trying to get a modern miniITX to fit inside,and somehow wiring it to the gas plasma display. I might try that if I someday get a busted T5200 with a working display. My thinking had been, by sticking an rPI in the 16 bit slot, I could have it emulate the net card, the sound card and add 3d acceleration to the video. A bit ambitious for an ISA slot, I guess.
-----
I was looking at tutorial videos about FPGA today, since someone mentioned them already. I think that *is* more like something I had in mind. There's a C64 "clone" made of an FPGA out there. So, hypothetically, an FPGA could be programmed to at least partially emulate a 3DFX (or other early 3d accelerator) chipset. I think.
I know I'm a complete beginner when it comes to hacking old hardware or building new hardware. That's going to be part of the fun of learning.