I've thought about MP in depth for quite awhile, there's no real way to implement it into the game in a way that both players could play within the same world, not easily anyways, since the game is based on taking actions in turns, and those turns happen to be very small turns, going between players to sync the world time would be a game-breaker.
There's other ways to go about MP though, for example instead of having two individual players on one map, you could have each individual player on their own map, and then have actions that can affect the other persons game in an "event" based system (trading, robbers, quests, etc).
I don't see a workable PvP version, maybe just for fun if you wanted to setup an arena with generated characters, with no real passive play, just a sort of arcade mode, even then the time system would need to either be replaced or you would have to let AI handle the characters.
Another way you could do it would be a dungeon master vs player style, similar to the event based system only you'd have a DM to play vs instead of another player in which you could reciprocate against.
Another method I've though about was a hot-seat mod where players could share characters over a server, whitelisted to your friends or whatever, could even have a public setting, anywho, you'd have characters on the server anyone or players from list X could play as, and they'd just download the characters folder to their PC, play, and then when they're done playing, save the character and reupload to the server, and mark it as available for other players who have access to that character, I believe players did this manually in the past, but it would be neat to see it handled via code and listed for others to enjoy. Could even handle different game settings and such for character replayability.
There's probably some other things you could do that I haven't thought about yet, but I think this encompasses what would be possible as far as MP is concerned... most of it requiring a massive overhaul to the code, I think the enormous elk team is focused more on content, functionality, and realism so I wouldn't expect to see an official multiplayer for awhile, if at all, but I can only guess
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