Ah the discoverable details within the portal games.
I know it probably treading old ground (many thought that with the second) but who else would like another Portal Game?
Maybe a portal prequel or side story.... could make ratman a playable character. Or could show off test subjects before chell.... will you die? will you escape? will you become a companion cube.
Or maybe they could turn the game 180o and have it from the machine perspective.... with all them cores still plugged in moderating you, while you set up traps and control turrets as you make humans run the gauntlet