This is unlikely to happen, as Clem has shared Linux Mint will focus on MATE, Cinnamon, KDE and Xfce.http://blog.linuxmint.com/?p=2000http://blog.linuxmint.com/?p=2038
Linux Mint 12 was using Gnome Shell with the Mint Gnome Shell Extensions (MGSE). MGSE has been discontinued, as Clem wasn't satisfied with it. Cinnamon was developed as a replacement for Gnome Shell and MGSE. So on Linux Mint 13 you can easily add Gnome Shell (install "gnome-shell" and "gnome-session"), but MGSE is no longer being developed by the Linux Mint developers.