THQ has confirmed that the Tyranids will get their own separate campaign in Dawn of War II:Retribution. There's a twist though in that when you play that campaign you'll only have one character to control, the Swarmlord (more details over at PC Gamer
on how this will work). It's an interesting choice and it will certainly mix up the game a bit as you normally have four units to control and I'm interested to see how well the AI of the units you don't control is.
I'm a huge fan of the Dawn of War II games and if you're burned out on Starcraft II
they are a very nice change of pace. Here's the teaser trailer for the new campaign.