I know sunless sea is a single player game but I believe that some sort of multiplayer would refresh the game entirely. Either by giveing the option to have a co-op game with a friend while online or having the sea populated by other real players ships.
The first option would be fun because you would have a partner in crime so to speak. You could both go different ways and meet back together to share rewards or maybe even your charts. Or you could go together thus improving your chances of survival with an extra gun and maybe the option to give one another supplies, fuel or crew if need be.
The second option is a long shot but would add to the game immensely due to the fact that their would be intelligent pirates other traders makeing the prices in some areas rise or drop.
I know this is probably way to big coding and story-wise to add to the game now but i though it would be interesting. I would also like to hear of others ideas for possible multiplayer.