Although there is no "roadmap," here are some rumors (from what clem has said, projects, wikis, specifically the mgse wiki, etc):
Linux Mint 13 will have some improvement to mgse, which will most likely feature one bottom panel and logoff/shutdown/etc incorporated into the menu, as well as possibly places in the menu.
This will presumably be done through a gnome 3 fork which clem has created called cinnamon (more info here: http://cinnamon.linuxmint.com
and here: http://forum.linuxmint.com/viewtopic.php?f=205&t=89411
) More on that later
It will definately be based off Ubuntu 12.04 and will be a LTS release, meaning it will be supported for 5 years (LTS releases are normally supported for 3 years for desktop and 5 for server, but for 12.04 cannonical decided to support it for 5 years)
The default desktop will, as I mentioned earlier, presumably be cinnamon, and there will either be a MATE version or MATE will be included. MATE has become much, much more stable recently and it works basically exactly like gnome 2, and these were clems concerns for releasing MATE seperately, so my guess is that there will be a MATE edition.
You can look around for Ubuntu 12.04 rumors, and the Alpha 1 is available for download. Here is an article with some rumors for it: http://www.pcworld.com/businesscenter/article/243541/seven_changes_to_watch_for_in_ubuntu_1204_precise_pangolin.html
Finally, here are some rumors for Cinnamon, most of which are definite (unless something completely unsuspected and radical happens with the project, which is highly unlikely):
It will definitely have a single panel
It will definitely have logoff/shutdown/etc in the menu
It will hopefully (and most likely at some point in time) have a places section in the menu
It will definitely be a fork of the gnome 3 shell (not all of gnome 3, just the shell). This means that all gnome 3/gtk3 apps will work perfectly, however gnome shell extensions will have to be ported (which is very, very easy), and gnome shell themes will not always work correctly.
The activities overview will be accessible by a button in the top left corner which will overlap anything underneath it. It is currently a clear-ish infinity sign. The top left corner hotspot also currently works, although I don't know about the future.
The overview might be changed, although right now it has no search, as all of the search functionality will be built into the mint menu, there is no applications section, only windows and themes.
It is supposed to be faster (at least IMHO), more stable, and much more integrated and therefore powerful than mgse was.
You can install cinnamon to Linux Mint 12 by simply installing cinnamon-session, or install it to debian sid, ubuntu (with gnome shell), or most other gnome 3.2 distros through github, although there might be some problems with distros which cinnamon wasn't tested for (It was meant for linux mint, so it most likely works in ubuntu and debian (only sid because gnome 3.2 is only in sid) as it will presumably make its way to lmde.
Dell XPS 15 l502x - Debian Testing 64-bit NetInst Xfce, SolydX 64-bit Debian Testing, SolydK 64-bit SolydXK Testing
Old Gateway Pentium 4 Desktop - Arch Linux 64-bit Xfce and SolydX 32-bit Sid