I wonder if a 487SX style approach would be feasible-- some sort of plug that deactivated the 8088 CPU, and then pairing it with a CPU card intended for a passive-backplane that could go potentially as high as like Socket 370.
As much as can be done on the accelerator board, the problem is that the moment you reach beyond the 40-pin socket, it's still an XT with a single PIC and the other limitations of the platform. External peripherals are always going to see only that. (Although I suppose seeing a 586-class CPU but only 8-bit bus slots will also look kind of zany)
Well for me the perfect "accelerator board" would look like this:
A bus extender with wired connectors to get a real 16 bit bus, so imagine 5 or 6 edge connectors with ribbon cables going into the card.
Adding a true 486+ class BIOS.
All of the other components needed so the system "sees" a real 486
Integrated Video with HDMI out with all of the bells and whistles possible including hardware MPEG, 3DFX, etc.
An integrated SD card hard drive
I still would want it to be able to use legacy cards and to use the motherboard for serial/para or on-board floppy.
I have no idea why I would want this to exist, but I would want it to exist. I think this is why people go Raspberry Pi route for these things because trying to get them all using legacy hardware would be impossible. An alternative would be a single other cable with a card edge connector for a Vesa Local Bus or PCI for a video card.
I know all of that is easy to do with another motherboard, but for some reason, for me, a card upgrade to the original bus just somehow feels like putting a porsche engine in a gremlin.
Chris