While I agree Darksiders probably has a lot more variety in the areas you explore, a lot of the variety in Darksiders are just things that Space Marines don't really do, and if it had you doing things like that, people would be confused why they are pushing crates around as a Space Marine.
The tile-sets probably could have been more varied, I don't know if a Forge World was the best place for visual stimulation(past the over-arching oppressive architecture). DoW2 has a ton of great tile-sets, I'm not sure why they didn't go for something more like that. Oh well, hopefully it will sell well enough to green-light Space Marine 2, and they can have you travel to different planets with all sorts of different environments.
But even with a ton of different environments, I am not sure what they could change up with the gameplay. I guess you could have some light RPG elements and squad control, sort of like the Deathwatch 40k RPG.