In my Star Clans campaign, I have them working directly for the O'er King (leader of the interstellar govt) but they're given wide latitude in terms of timing and specific actions, so I don't feel like I'm railroading them. I use random events (the tables from the old AD&D Oriental Adventures are great!) to plan my "future history," and player actions directly affect the flow, so I'm as often surprised by the direction things go as they are. Keeps it fresh for me as well.
Their current "mission" is to expand the Ardeni Kingdoms... and they've done very well. Each time they visit a system, I let them make their arguments with the local leaders. If their argument rings true to the locals (given what I know of the govt type and my notes on the leaders), I give them a bonus on the reaction roll... if not, they either get a straight roll or even a negative modifier if they misjudge the locals.
Of course, they could tell they O'er King they're done with that, and move on to something else, and I actually expected them to... Overall, the game did NOT go where I expected last night, but that's fantastic for me! They opted to continue their diplomatic & trade mission, and were very successful. And they all seemed to have a good time.
Now, if only the larger group would believe me when I tell them they can opt out of one game (Traveller) and still be included in the others (OD&D, AD&D 2e, etc.)... Yeah, I know, it's the equivalent of a "First World Problem!" We have TOO MANY players to run a single group! (More on that later...)