In the song at the end, GLaDOS says she doesn't need Chell anymore. And it's true: she's got those two bots for testing, now. I was guessing that the co-op initiative was essentially GLaDOS' happy ending--she can test forever and ever (and so can we), without ever having to deal with those intolerable and unpredictable humans. So unless they somehow insert another human into the mix, or take Chell's happy ending away and drag her back to the facility there's not much conflict there.
And the conflict between Chell and GLaDOS is also resolved, which was arguably the most important conflict of all in the game. Sure, GLaDOS doesn't like Chell, but she doesn't want her dead anymore, either. And without conflict, you don't have a story.
So if they want to get more story out of GLaDOS, they're going to have to start clean over. No Chell, no Wheatley, none of that. They'd need a whole new conflict with new characters and new problems, and somehow make it as heated and entertaining as the one between Chell and GLaDOS without treading old ground. Personally, I was satisfied with the ending, and I wouldn't mind if it stopped there.
But, this is Valve we're talking about, and they are essentially the Pixar of video games. They know GLaDOS better than any of us, so if there's any more story they want to share, I'm sure they'll find some way to get it to us. And if it takes a Portal 3 to do that, I'm open to it.