and the mode that's would work well with brink is a survival mode like the one in red dead redemption's undead add-on DLC. but instead of zombies just have waves of one faction coming at a team of 4 players in co-op. a bit like the survival in the challenges in brink but with more depth to it. obviously I'll let other people have there say as to how it would work. but the undead survival is very popular with many players and it would be an easy mode to develop.
you could also have the capture the brief case mode where both factions start at opposite ends of a map and the brief case is in the center of the map you then have to deliver it to a zone near where the enemies spawn point is. that mean you would have a good fight on your hands to try and get the case there. again what do you think? but as the game has no true single player campaign as such it could do with something else to give the game more gameplay options while using brinks great team work.