Hi,
during the design of Revolution Pi we do have considered all kind of possibilities, including PCIe expansion slots. There are several odds for such a solution from which I will only name the most important one: They are the opposite of reasonable priced.
With our current design, especially with the RevPi Connect you get a great connectivity which enables you to use all kind of physical layers. WLAN, LTE, MBUS, ZIGBEE, LoRa, BlueTooth, TinyMesh, field busses of all kind and of course Ethernet with TCP/IP can be perfectly used for remote internet access and other tasks. So what specifically are you missing?
Again: What is your aim? What kind of PCIe card are you aiming to? With the RevPi Connect you have several new wireless expansion modules. Except WiFi and LTE you get all the other wireless expansions for a RevPi Connect (will be available in August/September). What kind of USB Dongle are you talking about? If it is WiFi then I do not understand your requirements. The WiFi Dongle in our RevPi Shop is even less in height compared to your 180° USB to USB Adapter. The Teltonika LTE Router is the best solution you can think of for a remote IIoT system. It is affordable, has 2 independent SIM cards, diversity antenna pairs, offers WiFi access point ability and wired back up connection. It runs on 24 V and can be fully remotely controlled. I do not know any PCIe card offering such possibilities which are all necessary for a professional IIoT remote node.
This solution here https://revolution.kunbus.de/shop/de/lte-router is well-proven solid industrial grade technology and by far much more reliable than any USB stick or PCIe card solution. You can easily click it on a DIN rail and connect it to RevPi via WiFi or Ethernet cable. It can bring a whole cluster of RevPi core to the internet as it is not only a router but also a switch and a WiFi Access point too.