Cyan Worlds will fully open source all of Myst Online URU Live, essentially casting its lot with fans and other MMO players to grow and sustain this game.
Talking open source anything is well beyond my level of comprehension, but "fully" means what it says, and includes both server and client architecture. So this is a hell of a lot more technical than encouraging or relying on player created content, homebrews, mods and the like. They're seeking to create an actual development community, albeit among gamers.
For certain, many gamers are also coders. It's not mutually exclusive. But are they playing Myst Online, are they interested enough to come to it, is the game's code enough of a toybox for them, and is access to it enough of an enticement — all these are questions that Cyan Worlds will be answering.