I really, really think they should make a race mode. 4 players, 2 teams.
The maps would be a known quantity; the goal wouldn't be to solve puzzles, but to speed run through the map as a team in a race against other players.
You'd be able to see the other team through glass, like at the beginning of the co-op campaign, but not interact with them. Could also have certain parts of the race track projected on a huge jumbo tron screen.
You could either give both players portal guns, or, if you wanted to go the extra mile, re-introduce the paint gun, exclusive to this mode. Players would have to co-operate extensively to navigate the track.
If you really wanted to get crazy you could even re-introduce the green paint and allow for walking on walls.
Beyond that, you could even make the tracks partially randomly-generated, constructed out of basic components in different orders, sorta like F-Zero X on N64.
Something like that could have near endless replayability.