Bleh, looks like there going to throw it on half-arsed like a lot of shooters. Just a random DM mode because they're easy to make, like doom 3...
It does have a TON of potential though. I want to see a ton of plasmids/weapons and some team based mode. Reg DM is fun, but not when made wrong. Dosen't seem fast paced enough.
The multiplayer part of this game will focus on a prequel experience, pitting player against player as plasmid test subjects BEFORE the fall of Rapture. Players will take on the role of plasmid test subjects for Sinclair Solutions. According to PC Gamer, multiplayer mode takes place in 1959, a year before the events of the first game. There will be "story elements" and certain new plasmids that can only be used in multiplayer. Some levels will be set in partly recognizable areas from the first game. Players will have to unlock experience points to unlock new weapons, tonics, and Plasmids to customize characters.
Features
* Play as an original Big Daddy or kill him (he will act as a power-up)
* Collect Adam to level up and reveal more story
* Power up plasmids for more powerful attacks (fire, ice and electricity)
* Game Modes are: Free-For-All, Team Death Match and an undisclosed 3rd mode involving Little Sisters
* Use three load-outs to switch weapons during re-spawns
* Ten player matches
* Pre-game menu is a 3D modeled apartment
* Personalized character
* Each character has their own melee weapon (e.g, the housewife will have a pan, the business man will have a golf club, etc.)
* Your avatar is represented by the person you play, the more you splice, the uglier you get
* Plasmids are more dynamic in how they react with the environment (If you had Incinerate! and lit an oil slick, you could put it out with Winter Blast
* In the fourth BioShock 2 Podcast from The Cult of Rapture, there is mention of there being ten official maps shipped with the game, hinting toward DLC maps later on
Taken from the bioshock wiki. So no, it is not just tacked on deathmatch.