A problem I had years ago was, I was living in an apartment that didn't even have a phone line. So I used 3G (4G was there but just not working at all) because the house Wifi was useless, in fact in the whole area this was a problem. So I started getting interested in using public Wifi (Freifunk) with a big antenna as upstream, and also provide it for the house eventually. But since then I realized that even on a normal Linux computer, configuring/routing multiple Wifi interfaces is absolutely non-trivial.

Eventually I gave up, but I bet Mesh Wifi are seriously becoming a thing in dense cities. Also some off-the-shelve routers allow 4G-USB-sticks as backup options. Fully automatizing this would be so awesome!