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?
I use them for additional network adaptors in one host.
In another I use them for NVMe adaptors for an ancient MB that predates the existence of the m.2 interface.