Space Marine's recently added co-op horde mode already has additional content on the way. Latin-Dalek-impersonating Exterminatus mode originally added two scenarios for free, but THQ are preparing to release another chunk of content called Chaos Unleashed. Keeping Chaos on a leash is like keeping a cat on a leash so this is a sensible move but what does it add to the game? For the price of "800 Microsoft Points or equivalent", them being the world's major currency apparently, Chaos Unleashed let's you fight swarms of Orks and Imperial Guard as a ruddy great Chaos Marine.
The inclusion of both Orks and the Guard is suggestive of at least two new scenarios and there will indeed be multiple arenas, new boss rounds and new Imperial enemies to rip through. On top of that, there are three new multiplayer maps, which is nice.
I'm all for a bit of Chaos Marination. My first and only 40K army was made up of the blighters, with my modelling skills extending to gluing an extra limb on every third Marine and thinking to myself, this is totally the most chaotic thing.