λ the β-Redex Reducer is right, that's about it, but if you want to stretch the imagination, you could conjure up future extensions.Can you add any others? It’s quite an interesting approach
For instance the Librem 5 doesn't have FM or LoRa. Maybe one can imagine taking out the modem and plugging in a FM card (with some kind of antena ?), or a modem+FM card, or take out the Wifi card and plug in some LoRa gear, or whatever.
I'm not sure the interface would make it easy (would the software have to send an AT command to dial an FM station ???) and the effort could easily be much more than it is worth however you look at it, but crazy people make crazy things, so who knows...
Looking at docs it seems some interfaces are MDIO or I2S, but I don't know... I guess it would make even less economic sense than technical sense.