Depends on how flexible the world-controlling engine code is in relation to the content of the actual game world. I once imagined something like this and as long as enough work is placed on the fundamental mechanics of inter-server operation then I can see it as doable.
Basically they need to have a set of rules with enough variables that can allow for the individual server environment to change sufficiently enough to make the different worlds interesting while still retaining a 'core' code that can handle the transition well for networking to different servers. Like say the effect a wizard using a wand to casting a 'Flamethrower' spell also being able to work with a steampunk 'flamethrower' or a sci-fi flamethrower. For world/quest building it would be great if they had simple rules/tools with the possibility of becoming something enormously complex like a game of Go.
World building tools that allowed player add-ons while preserving connectivity could create an enormous amount of player game support like the way Crafthub does with Minecraft for example. It would take a lot of planning to avoid the 'need this add-on, and this add-on, and this add-on' to play though. A game engine that allowed add-ons to modify the environment while not breaking compatibility between versions would be very nice but would require some very heavy thought to create..
I actually downloaded Unity last night to try playing around with something like that before I saw this article and said 'huh. Great minds.'