At this point, I have no information about the plot of Myst V. But I thought it might be fun to list the Ages we're pretty sure are going to be IN the game itself.
Since, it appears, that much of the unused Uru:Live content is being used for Myst V, many of us have recognized location screenshots that have been popping up online here and there.
What it looks like so far:
1) Negilahn - The "Jungle" Age. The background image used on the Myst-Tree.com site is Negilahn concept art from Stepehn Martiniere's website. Myst-Tree.com has been verified through official channels as being Myst V related...
2) Todelmer. The only real mention of Todelmer is a graphic at myst5.ubi.com called "Todelmer Scope."
If you aren't familair with Uru and it's current fan-run online cousin, Until Uru, there are two inoperable linking books in the City Museum -- one links to Negilahn and the other to Todelmer. Obviously, these two ages were meant to be released soon after Uru:Live was intended to begin.
3) Noloben. Fans have been salivating after Noloben since the first Noloben screenshots went up, what... two years ago? We now have a new screenshot of Noloben at myst5.ubi.com. (Noloben is also mentioned in a note on Douglas Sharper's desk in Uru.)
4) Laki. The music playing on the Myst V site right now sounds very much like Jack Wall (Myst III, IV and Uru composer) and is entitled "Laki Arena." (Another unreleased Uru age.)
5) Tomahna. You can see a screenshot of Tomahna HERE at the D'ni Jazz Club site. Looks like the telescope wasn't the ONLY thing to fall through the Riven fissure. (Speculation: Is this inside the volcano?)
6) Also at the D'ni Jazz Club link above, a screenshot labelled "Node." Many might recognize this location from one of the earliest 3D tests the Cyan programmers did for what would eventually become Uru. (A video test of this location is available as an Easter Egg on the Myst Masterpiece Edition for MAC.)
For an interesting list of unreleased ages that would have been associated with Uru but now could easily become part of Myst V, check out the D'ni Restoration Council site right HERE .
Please feel free to comment and speculate!