Yup. If coop is bound to stay that simple, it will turn bland and stale in the long run. It's already starting to become a routine, actually, even though I'll admit the lack of different maps doesn't help.
I forgot to mention Survival. I spent so many hours enjoying that mode in Ins2... In fact, appart from Outpost, I really loved all coop game modes. I just believe that Outpost might be fun in larger maps. Would possibly avoid the swarm of AIs in closed and tiny spaces with little to no cover as in Ins2. It should instead be called "Stronghold" and the cache to protect would sit in a building with many windows. Insurgents would come from every direction. Now that would be something.
The only one that is redundant is Conquer. When you're alone in the server you just can't win, and it's kind of the same as Checkpoint anyway.
I'd like every single coop game mode from Insurgency 2 to make a comeback in Sandstorm, with the exception of Conquer. I'd be a very happy player if coop had that kind of variety in gameplay.