It'd be too easy to say "Perhaps the ending has not yet been written"
, but I'm fairly certain there won't be any more Myst games. Myst V: End of Ages is meant to wrap up the story of Atrus and his family. Yeesha's legacy is explored in Uru (especially the main quest), but for reasons I can't explain with spoiling the plot of Myst V, it might be better to play Uru before Myst V.
Given Cyan Worlds' tough financial situation, it might be a long time before any new content will be made available for Uru Live, all of their existing Age content is currently free of charge.
It will also be a long while before the fourth Myst novel, Book of Marrim is published.
