Zombie games can be fun, but I would prefer that the gametype stay in FPS games that aren't focusing on a storyline. One of the main hooks in Brink is that this is the near future, global warming happened, and people are struggling to survive. There's an air of plausibility around the whole thing. It really drags you in, so you can feel like you are one of the people fighting on the Ark, and care about the outcome.
The moment you throw in zombies, you lose a lot of that. Yes, games focusing on zombies can create the proper atmosphere to get you into it, but Brink isn't a horror game.
I would welcome a DLC containing new guns. I understand that games can sometimes go too far and turn their whole business plan into making micro-transactions, but i just like having more options when it comes to weapons. Choosing your gun is one of the main tactical decisions you have to make, right next to picking a body type.