I'm waiting for the two stories to conflict, where you play one chapter from each character's point of view depending on the game. Ex: You play as Gordon along side Chell (or possibly a new subject) in HL2:EP3, and vice versa in Portal 2. That would be cool, especially if they break up at certain parts so you don't know what the other person is doing unless you play the other game. Can you say epic marketing technique?
Gordon and portal gun would make the best of friends, I can only imagine the fun of using portals to portal combine soldiers in to a fire (or off a cliff, or in to a room filled with exploding barrels).
But valve has said that there will be no portal-life, That makes me sad.