Confederation Chronicles: An SF Campaign Premise -- Part 01

My old gaming group used play a Star Hero campaign, which is the main influence for this idea -- coming up with a (mostly) plausible basic premise that allows for multiple types of adventures for an ongoing Science Fiction campaign. I'm hoping to recreate it with the toolkit and setting elements in Sine Nomine's excellent series of Stars Without Number RPG books.

But before you put together something big like this, it helps to list the goals for the premise.

Goals of the Campaign Premise:

• Combat - no bones about it, we love combat. But because we do, just another combat scene isn't enough for us. We look for different challenges, different restrictions, different locales, different scenarios, and different stakes for every combat scene that's sprung on us. That pretty much makes a military aspect a must in the campaign.
• Starship combat - related to combat, but a different experience altogether, we want the opportunity for ship-to-ship combat, whether we're talking capital ship combat or fighter ship combat or ships with boarding tactics or some mix of everything..
• Strange New Worlds - the Star Trek influence of having a new race, a new planet, a new wrinkle in the setting exposed in almost every episode stuck with us as well. This suggests a rationale for constant travel in the setting.
• Space Opera - inspired by Star Wars, the Lensman series, and elements of Babylon 5, we also looked for things like 'space knights', and 'fragments of a fallen star empire', and 'lost technologies', and 'god-like alien entities and artifacts' mixed up into the stories and adventures of our characters.
• Cloaks, Daggers, and Senators - equal parts Babylon 5 and Alien Legion, we also like to have that feeling of politics and espionage and compromise and brinksmanship and betrayal. Most of the time, this is backstory for the hoops that the PCs go through, but on occasion they do get to impact the interstellar (and sometimes intergalactic) stage.

Of course, at the same time, I'd need to have a rationale that not only allows the players to portray a team of PCs that work together, but also has a rationale to (a) shift characters in and out; (b) introduce replacement characters; and (c) perhaps send in completely separate character teams connected to the same storyline.