We already have seen that TWI does not have the ressources to pull off a full sized campaign. RO2 Singleplayer was - sad but true - wasted ressources desperately needed for important things.
If there were more people working at TWI, let's say they were the size of Treyarch, then they definitely would do good to do a campaign. No CoD run and gun, maybe something like Brothers in Arms, which was on high difficulties a real challenge, especially the enemy AI was formidable.
Call of Duty Black Ops had a few tunnel levels which were intense. If mappers are capable of bringing merciless tunnel fighting into the maps, that would be indeed worth something.