People want more daily ore refinement. Cryptic wants more social play and more interaction with strangers, I'd assume. But PUGs (Pickup Groups) have risks.
So here's a thought:
Every time you successfully complete a queued PuG mission (tracked by an invisible buff placed when a player queues; fleet action, STF, whatever), your daily refinement cap for that day is increased by 250. Now this is less than the dilithium you'll actually get in most cases where dilithium is awarded so there is a diminishing return on it. But you can refine more than you could otherwise by participating in PUGs and it generally incentivizes faster and non-dilithium awarding PUGs more than they are currently, from a practical perspective.
I think there may at least be something to the idea of increasing your refinement cap for a given day based on the type of activity you perform.
And in retrospect, that's probably also what I'd look at doing with STF optionals. Shift the existing optional rewards all over to be added to the standard completion award and make the optional completion purely about getting a boost to daily refinement cap.
This addresses dilithium starvation and over-eagerness on optionals for casual groups while giving hardcore players a strong incentive to complete optionals.
So basic STF rewards would become what they are currently with the optional completion, no optional necessary. This gets people closer to the daily refinement cap. And then optionals only matter to people who actually hit the daily refinement cap, making them properly targeted at a more hardcore category of player.