I think the agent system needs some sort of streak breaker, so that every 50 missions, 20 scenarios, 10 blue chests (lair & dungeon separately) or 5 raids you’re guaranteed to get a dossier if you got to that number without a drop. (numbers were chosen arbitrarily)
As an alternative, an increasing chance like weapons from caches would be ok.
Personally, I’m now past 100 scenarios without a drop and if I didn’t enjoy playing scenarios as much as I do, I’d be up for punching something.