Could you tell us a little more about your PCs. What is the rebel rebelling against? What sort of items is the smuggler willing to smuggle, how ruthless is he? Who does the spy work for and why? How greedy is the Ferengi, what does he usually trade in? All characters need a reason to do what they do.
A campaign could center around opening up trade in a new frontier of space along the border with any major power you like. In what era are you wanting to set your campaign? Pre-Dominion War , during, or after? The Cardassians are really weak, they have near a billion dead on Cardassia Prime and their fleet was gutted by the war. Every other power in the Alpha Quadrant has lost about half of its ships and men. Just give us a little more info and the folks around her can really help you out.
"Retreat?! Hell, we just got here!", annonymous American Marine, WWI
"Gravity is a harsh mistress....", The Tick