I don't know if this is mentioned before or planned, but just building a large relatively bland, blank landscape that you can allow players to edit and build environments in so that it allows a large playing area for those who want free roam open world style gameplay and lighten the load on yourselves. Maybe monitor submissions and newly made areas/base/cities/landscapes so that no terrible crap is made.
I'd like to see hub cities/towns, they dont have to be complex just a few buildings/vendors and other interactive things, also a really simple ground game mode in these cities/towns where say you do missions like having to steal things from a building and smuggle them onto your ship.
other players may be trying to stop you getting it onto your ship so you can transport it, but every character gets a handgun/musket which is really effective but only used sparingly as it may have only 5 bullets or something, but the fighting is one hit kills like Red Orchestra. but then you can run away from pirates in town and make a daring escape.
This man speaks gold, I've seen player developed map design in it's fullest flourish in my earlier days, to iterate:
I used to do alot of RP on private WoW servers, and we would use the tools to take existing areas, even flat andu nused landscapes to completely built up towns, capitals, towers, villages, anything your heart desired. It was glorious, I've seen some amazing landscapes being made that way. If there was a pair of MUSE along with a board of CA's to take care of selecting good and bad submisisons, this would put the burden of open-world map making on larger shoulders, so to speak.
I can however see alot of problems with it, instead of having everything secluded to the team itself, who works internally and actively (I assume) in NY, there'd be a larger team, independent of state or nation, with less ability to communicate simply by raising your voice and going: "HEY, DO WE WANT LAZER GUNS HOWARD?". It also completely ruins the current teams' duties: Instead of creating it on their own, they would have to shift their values to judging and overseeing submissions.
Another thing would be submissions, it requires that people actually want to go in and do something actively. For larger games and populations, like the Steam Workshop fanatics, it's a great idea. Simply because they have so many people playing and developing, thinking it realisticly with GoIO. We don't really have the worlds' greatest population, and putting another job on whoever would get that "Overseer" task is just stressing out a tough development even more.
Finally: the tool itself, unless the team put alot of effort into making an easily accesible and useful tool for dummies like me to use, only a very select few would be able to understand and work with it. I don't know much about programming and graphics' designs, but I do know that you have to spend a while to understand what you work with. Making the tool would either be too time-consuming, and just giving what they work with would be too complicated.
I love the idea, I really do, but it may be abit out of hand. Who knows, maybe it isn't as hard as I predict, and we'll all be map-making monkeys within a month.