Opera has been spearheading a lot of innovations in browsers. Likewise, one can notice that Apple's OSX Lion may have taken some inspiration from Gnome-Shell. Most notably the idea of a flexible number of workspaces. Of course, no one knows whether this is really where they got inspired, but hey, at least some of us saw it first in Gnome and then in OSX. So I think being the spearhead of innovation in desktop environments and leading the way in innovation, would be a great new goal (or mission) for GNOME. It is well set up for that too, because it doesn't have nearly as much at risk as Apple or Windows have. This is a typical benefit of being small: more agility.
Also, I'd like to point out that GNOME has, in my view, achieved more than what they said out to do. A *working* free desktop environment? Gee, this is more than just working! I personally prefer it over the OSX Lion desktop environment I use at work -- and Windows currently isn't really a player.
GNOME has worked hard to make it to the top: now go and lead the way, at least for a while!