Purely a guess but don't be surpised if the 3rd DLC is another map pack...
Horde mode has been ruled out
Co-Op has been ruled out
New game modes have been ruled out
Unless ND wants to bury themselves in a big hole with season pass holders, the 3rd pack better not be a customization DLC pack (more lame hats, more goofy masks, colored guns, colored backpacks)
Another map pack
judging by it, it is a good chance it is gonna be a map pack + customization.
maybe if we wish hard enough, there is like a 0.001% chance of it being co-op/infected related, but I personally am assuming it will be more maps.
June, 2013 -- When ND listened to crying AGAIN!
A competitive mode.
If you dig through the DLC announcement posts, Eric replied that new modes aren't planned. The only mode that I could see people paying for is Co-Op. If it's competitive modes, people are going to be up in arms that it wasn't free.