I would like another Portal.
See, I liked Portal 2 or Bioshock 2, I'm more of the movie mind, I've come to see a sequal, I'm expecting it to relate to the previous one, advance the characters, add new bits but not so naive that I'm still expecting it to be as revolutionary as the first one.
I like the world of Portal, but I'm not sure how you'd squeeze another one out without making it boring or similar to the second one. I would rather not have another Portal if it was bad, but I doubt Valve would make a bad portal either.