I would like to begin with to state that I really enjoyed the game. The campaign was short, but nonetheless very enjoyable mainly thanks to the excellent narrative, great atmosphere and the fun action that the various weapons iconic to WH40K brought.
Regarding co-op, that too is initially a lot of fun. However, once you're properly upgraded, unless you're in a full squad and playing on No Mercy with Friendly Fire turned on, there's not much to work towards - hence the idea of more trophies.
Putting in new trophies - either free or via paid DLC - that would focus on tough co-op based challenges, where the settings would be locked on NM and with FF enabled, would definitely increase the incentive for me to keep playing.
I have no idea whether you have the resources to implement such an idea, but I feel it's one of the more realistic options that would require a minimum amount of changes.