Portal 2: an "in-game editor!" According to a PR statement from the company, the next DLC is currently "in development and targeted for release early next year."
Apparently some form of community functionality will be attached to player creations, allowing other users to "view, play, and vote" on various levels. Outside of some cursory details -- the editor will allow players to create and share both single-player and co-op levels -- little is known about how editing will work, or if the DLC will arrive on both PC/Mac and consoles.
With Steam support on all but one console, the Xbox 360, we wouldn't be surprised if the second Portal 2 DLC doesn't arrive everywhere. But then Valve head Gabe Newell indicated differently in a recent interview, so we just can't be sure. We've reached out to Valve for some clarity.