The pinout is already standard, you can use mini-PCIe (common in older laptops) or the more recent M2 slot (which is the same PCIe but in a different form-facto

The pinout is already standard, you can use mini-PCIe (common in older laptops) or the more recent M2 slot (which is the same PCIe but in a different form-factor). In fact some boards already come with 2 M2 slots, so nothing prevents you from using the second one for a wireless card if you so desire. Bluetooth on the other hand is low-enough bandwidth that a USB adapter works fine generally (USB Wi-Fi adapters also work but USB uses more CPU which is a problem for high-bandwidth networking). Personally I just don't see the point of Wi-Fi for a stationary machine anyway - you're wasting valuable spectrum for something better served by an Ethernet cable (sourced from a powerline adapter if you're really desperate).