Well, seeing as mobile multiplayer is still a thing, it would be great if we were able to get some sort of support.
For instance, an online server or multiplayer lounger where people can hook up to with their maps (with the option to password protect them) for other players can log on to.
Problem with that though is that mobile sessions are very unreliable as most games are played "when time allows it" and thus the host may be gone all of a sudden.
A solution to that would be to have the maps hosted online so it's not user host dependant, but that would require server support which also means maintenance costs for the devs. However, this could be fixed by having an IAP system where you pay $1-5 to have your map hosted on a Terraria server for a month or something. Joining online games would be free, just the hosting that you pay for.
Seeing as there wouldn't be alot of data transfer involved (small file sizes), the cost to maintain such a system (dedicated server) should not be too high.