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?
You can use a SAS controller or 10G ethernet card in the 16X slots. If your motherboard support PCIe bifurcation, you can get an adapter card to use 4 NVMe drives in a 16X slot. You can get controller cards for older interfaces like firewire, serial, and parallel that will use a 1X slot.