Could be that something along these lines has been suggested previously, but if it has I can't find anything similar at first glance so I'll go ahead with voicing the idea I had recently.
In short, you can join a server as a crew member not linked to a ship, so as a castaway. This comes in turn with gameplay that allows for removal of crew members (mutiny, walking the plank or choosing to leave) and hiring of new members on to your boat if there's a space. Could also allow for castaways to commission a boat at any dock in the game should there be the server capacity, so the number of boats on a server, as much as I would like to see it increase a little, would remain the same.
So giving some more thought to this, I was thinking the maximum number of isolated players that can exist would increase, this would be server capacity dependent, but I'd like to think there's one additional player allowed for each boat on a server, so I believe at present that would be 5 additional pirates irrelevant of boat size. Castaways could spawn in on non-outpost islands or sea-posts with an empty standard rowboat and no supplies, only what they can forage. If they die, they respawn on the last non-outpost island they set foot upon, but without the rowboat, or a random nearby island if that one is now occupied by another crews ship. Any ships encountered could recruit the castaway if they put a scroll on the captains table asking to join and a majority vote accepts their application, adding them to the crew roster. Could be interesting to increase maximum capacity of crew members on all ships by one to accommodate this, but those you can invite from the main screen would be still capped at the current numbers, so you couldn't invite a fifth player in from the title screen on a galleon for example.
When it comes to removal of characters, a lot of this would follow similar logic to brigging players and be done from the crew management menus and follows the same majority vote logic. To prevent flooding the servers with wandering individual pirates, could say that you can only evict a pirate, or conduct a mutiny, if the crew is full and then put removal/addition of new crew members on a one in game day timer. When players are removed, they're advised they have been removed, but stay on the boat and only transition from a green crew member to a white non-crew pirate after about 10-15 seconds, allowing them to prepare for combat if that's the direction it looks like things are going, or to walk the plank if they want to get in to the role.
Mutinies could be a captaincy only feature and actually cause a different set of circumstances to occur, whereby the Captain and whichever crew member initiated the mutiny fight to the death. If the captain wins, they remain the captain and mutiny is no longer an option, either again within that session or for a number of in game days. If they lose, they become a crew member and can be voted off the boat. The victorious crew member then becomes the captain and can swap the ship out for one of their own at the shipwrights/ship cosmetic chest at the outposts if they own a ship of the same size, or the boat simply becomes un-captained.
Could add combat challenges amongst the crew as part of either of these man-overboard options, but the option to turn on friendly-fire is probably a suggestion of its own. Anyone being voted off remains on the server and can seek revenge on their old crew, either solo or by joining another crew. If a crew member is removed, there is also nothing preventing the crew accepting them back on board via the crew hiring mentioned before. To avoid player caps increasing beyond the threshold though, the player still takes up one of the player slots on your ship until you adopt a new crew member in their place. Players removed would also take their voyages they placed down with them, in their current state, to prevent people getting others to put quests down and then kicking them off of the boat.
Commissioning a boat would be a smaller topic of discussion, but essentially if you joined a server as a castaway, but there's a slot available for another ship on your server, you can respawn your boat (owned or not) by commissioning it at a shipwrights for a fee akin to repairing an entire damaged ship via captaincy. This would then respawn the boat as if it just sank, so you would take a mermaid and be returned to a freshly stocked ship at a random island ready to set sail.
There are probably other things to work out that I haven't considered, and whether this is possible at all greatly depends on whether servers could handle a few additional loose players, but I feel this could add a fun dimension to the game and some piratical elements that are missing at the moment. I'd be intrigued to see what other people's thoughts would be on this though.
