Given Apple’s current locked-down trajectory with the Mac, the Mac Pro was gonna die eventually, and it’s for the best that it does given it was reduced to little more than a massively overpriced Mac Studio grafted onto a useless PCIe backplane; a $12k grift, basically.
PCs at least are still modular and expandable; for now.
I’ve seen people mod old cheese graters, but almost all of them in my opinion ruin the case. Those british kits are the most common and pretty bad.
https://www.tomrei.com/2016/11/apple-power-mac-g5-casemod/
This one got pretty close, even though it needed to be painted and didn’t do the rear holes. Other decent ones keep the old expansion slots and just make another hole for motherboard io.