It probably comes as a surprise to no one, but Valve Software have confirmed that further DLC for Portal 2 is coming this summer. Valve's Doug Lombardi told Fast Company:
"We will be announcing some DLC for it, so folks can keep a look out for more content coming this summer. We'll be putting more details out about that in a week or so."
What are you hoping for? More co-op levels or perhaps a brand new chapter? We're just hoping for something that will surprise us...