It's hard to check success rate on item collection

I disagree with this idea. The people that need the collection are mostly player that will never make it to the point they can run elite missions.

A causal player or some one returning trying to catch up are the ones that need the help. To get to where you can run elite missions you need a ton of agents that your able to upgrade and need to run a ton of mission to open up the quest.

Common quest should reward 1 collection item. the higher level ones if they don't want to give out more collections could give agents or coin as an added bonus.

Common = 1 collection
Uncommon = 1 collection and coin
Rare = 1 collection and an agent
Elite = 2 collection