Valve will never make a Portal 3. There is just nowhere to improve.
The star of Portal 2 is its single player campaign. The plot and dialogue are both excellent, and the visuals are varied and detailed. The puzzles are fairly easy, with a smooth learning curve, and do a good job of avoiding being frustrating. Nor are they repetitive, but constantly find ways to intrigue you. The campaign is not too long, but deserves being replayed, and as I will come to explain, for some more dedicated puzzlers this may just be an introduction to the game.
The co-operative campaign is also excellent, and more challenging than the single player - as long as you can find a friend to play it with. It is equally varied and amusing as the single player, but adds an extra requirement for co-operation which is a great addition tio the game. Again, however, it is quite short.
Here, however, is when the more recently released feature (available only on PC) kicks in: the Perpetual Testing Initiative (PeTI). This allows anyone to build their own test chamber out of whatever crazy ideas they have and upload it to Steam for others to play, and equally to play maps other people have posted themselves. And given that so far over 200,000 maps have been created, well... that's a lot of testing! And that includes co-operative chambers as well.
The standard map builder you get is wonderfully simple - the interface is perfectly intuitive, and you can stick together whatever elements you like as quickly as you can think of what you want to make. There is a second editor you can choose to download, much much harder to use, but which grants you all features that the game developers had themselves. There are a few such collections of maps on the PeTI to download that rival the standard campaign in terms of excitement and plot detail, and I would highly recommend these.
Overall, Portal 2 is a game based on a fantastic puzzle idea. The standard campaigns showcase the idea perfectly, with amusing and engaging plot and gameplay. And if you think that they were a little too short, then there are plenty more great new puzzles to play, and the ability to create your own limited only by your creativity.
I wonder if any Portal 3 could even attempt to improve on such a perfected game.