We're going to be very busy for a long time yet with just the original game, and we currently have no plans for any dlc/expansion with original content - but there's no rule that says we're the only ones who are allowed to make Myst things.
There's nothing stopping you guys from making Myst-themed content! (We've already seen plenty of it here on the forums from some very talented people). If you have passion, you can make whatever you want.
... and of course, If you decide to make anything, we'd love to see it.
