I have 2x PCIe X16 and 1x PCIe 1x slots that are not being used. Given that Linux has extensive hardware support, there are probably users with interesting PCI card usages
Aside from traditional usages like network/wireless/bluetooth/sound that can easily be used as USB (or built in advanced sound support in the MOBO), what are your use cases with PCIe?
This is a really interesting question. I remember a lot of very niche PCI (not PCIe) cards from the old days.
My current-day boring use-case: Multiple GPUs that have never once been used for gaming. One is an A2000 card that does video transcoding and tone mapping for Jellyfin. The other is an old Quadro card that handles encoding/decoding for my NVR.