.
If your thread was Closed, nobody would be able to crash land a spaceship into your plot without your consent, and you'd effectively be the Game Master for that specific iteration. However, if somebody made their own thread that basically duplicated your plot up to a certain point, and it diverged from yours with the hypothetical spaceship crash landing, other players would be able to take part in that story without interfering with your plot.
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The common ground would be "Given an infinite amount of space, everything that can reasonably exist will exist, and it'll be duplicated even further out, with some of the duplicates progressing at different rates of time." Beyond this, each thread would functionally be its own spacetime. They don't need to focus on specific locations, and could even be centered around certain objects, people, or other things. Whether a thread is Open or Closed is up to its creator, but each thread in the subforum would be part of the whole that is the Expanding Horizons. Nobody would be forced to roleplay with anyone else, and everyone can split off into their respective threads if they can't find a common ground. That way, everybody collectively decides which stories they want to participate in, with all contexts being agreed upon as they're presented.
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You could assume that, but the Persistent World would serve as an easily accessible repository for all of that information. The Closed threads would just be your way of publicly roleplaying events that take place in a specific portion of the infinite universe. If they wanted to, other people could make a divergent version of what transpired in your Closed thread, or they could request to be allowed to participate in its development.
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I believe not copying other people's roleplays and characters is conventional roleplaying etiquette. However, if those roleplays and characters existed in Expanding Horizons, this wouldn't be a problem, because the setting is designed to accommodate potential duplicates.
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Rather than risk irreconcilable conflicts between a GM or roleplayer, and someone that would like to duplicate their roleplays or characters, I think having a framework with the initial assumption of such permissions being granted would be ideal.