Yes, I know this isn't the right place for this. but I can't find a better place
Here's what I think Valve can do for portal 3:
Make it a MMORPG.
Instead of being tied to a specific character like Chell, Atlas, or P-Body, you get to pick from the vast repository of test subjects in stasis (or to be precise: you get to design your own). You can team up with up to maybe ten or so people in order to solve puzzles and stuff.
But one person plays the role of the evil A.I. (probably get to pick a skin and use a voice modulator). his or her job is to get the group to complete the tests, but kill them when they leave the testing rooms. the A.I. person would have all sorts of bot and traps at his (or her) disposal, and can monitor all going ons (to an extent: ideally limited to using the cameras and rough proximity sensors once they leave the course) and the A.I. person would have some control over the layout of the map (enough to make it unpredictable, but not enough to make it impossible)
This could also possibly translate well to an offline multiplayer for the Wii U, since the gamepad can be used as a screen only visible to the A.I.