I've been going around the forums lately and notice that there is a couple of threads calling for a Zombies style DLC and personally I have always loved Zombies but I don't think it really suits Brink.
But I do feel that a Horde type mode would suit it. (Horde from Gears of War 2) This could lead on nicely from the end of the resistance campaign, Nachieve returns bringing with him hordes of savages who had survived the floods. This would be great fun but also tie on to the story line, extra Campaign missions could easily be attached to the game mode.

The game mode could feature many of the usual defence objectives like constructing MG nests but with the addition of objectives like destroying the savages transport ships and camps on the shoreline. The resistance would now fight on two fronts, defending against the invading savages and assaulting the security while their defences are stretched. The security will have the hardest fight, Defending from two sides they must now rise up and restore order amongst the guests and eliminate these invaders.
For the survivors of the flood the Ark would seem like a place of luxury even for the guests, being given water and having somewhere to sleep is better than nothing. The survivors seek revenge on those who hid from the rest of the world in their Ark. The Survivor objectives would be to seek and destroy the residents of the Ark and then take their technology for themselves.

5 man squads would be a good size for teams and a unlimited time span would really allow players to rack up the points and go head to head against friends.

I've got much more stored away here so what is your feedback on this post?

You, sir are a genius!

Also, zombies can fit anything as long as they have the same graphics style. They're just melee-only enemies with either lotsa health or lotsa speed to balance out. Usually health.

I agree. I like to play the last challenge - Tower Defense - however, I sometimes feel that the map isn't very good if you are playing with four people. But man, is that a great challenge.
Gears of War 2 changed gameplay with Horde mode and ever since then other games (Halo: ODST included with Firefight) have honed in on the success.
For once, I think Brink would be able to incorporate this type of gameplay without it feeling too distant from the actual "story mode."

On spotting this topic, I immediately thought of the 4th Challenge, as ViralRobby put forward. It's already been proven to work in Brink and, with a little polishing, could be great.

Wahoo! I have a good idea!