Even after all these years and several games, there are so many mysteries and unanswered questions from the Half Life series. This is all intentional, though. Valve understands the power of mystery and imagination. The fact that there are so many hidden possibilities is one of the things that draws us in to Gordon's story, and Portal.
My point being, don't assume that there will be another game just because there is more to figure out. Personally I don't think there should be another Portal. Sometimes it's best just to quit while you're ahead. Valve is obviously extremely creative, let them come up with something new and leave Portal as the masterpiece it is.