Portal 2 may only be a week old, but if you've already finished it - and these days I feel like the only person on the internet who hasn't - you won't have too long to wait until there's fresh content for the game.
Valve's Doug Lombardi has told Fast Company that the first Portal 2 DLC will be "coming this Summer", and there'll be more comics and trailers (presumably more of the "instructional" style) to go along with it.
The addition of new levels would be the most obvious candidate, but it'd be neat to see something added that makes people replay the game again. Tweak, add or substitute some of the traps and components found within the game, that sort of thing.
Portal 2's Creators On Crafting Games Through Experiential Stories [Fast Company]