Sometimes when I'm racking my brain, trying to come up with adventure ideas, it helps to think about the basic elements of space opera--or at least, the elements that I believe are the biggest dramatic story turn-ons. So if I get stuck, I remind myself to start small and build up on bare bones. Here's the bare minimum that I try to work with.
FOURPlayers. Yeah, we do just fine with three or even two, but four is a magic number because real group dynamics come into play. No longer is it the dynamic duo or trio, it's a fully realized team. It's much easier to diversify skills with at least four participants (besides the GM).
I go by the acronym A.R.M., which stands for alien, robot, monster. That could mean there's one of each or three of the same--or some combination thereof. Doesn't matter if they're friend or foe, but weirder is always better.
These creatures don't need to be instrumental to the plot, they can be encountered incidentally or as foreshadowing for a later adventure. But space opera is nothing if not exotic, so this is where I can turn up the dial on "fantastic beasts."
Hopefully every game has at least one thing they've not encountered before or at least has a unique spin on an old trope.
At least two destinations are required--could be planets, space stations, etc. But space travel needs a Point A and Point B to be interesting. Doesn't matter what those two places are, but preferably they require travel in a spacecraft of some sort. I always have at least one scene/encounter in space. ALWAYS.
I tend to ping pong my players between highly contrasted worlds (high tech vs. primitive, utopian vs. lawless, paradise vs. unbreathable hellscape, etc.).
A Big Bad Wolf! There's gotta' be at least one scheming maniac, super-powered warlord, evil space wizard, etc. This usually helps to not only give a climactic battle, but to flesh out some of the plot with motives and minions (henchmen/bounty hunters, etc.) that might be encountered by the PCs, as well as other story-centric elements that creatures in the A.R.M. category aren't always intended to exploit.
At least one thing that's totally out of my control. For one game I gave the PCs a mutagen stim, with a random table of possible mutations. They were encouraged only to hold on to it until the last half of the game. It surpassed all expectations as they really went for it with roleplay and it became the best part of that game.
What about you?
Again, these are just the starting positions, but they generally get me off to the races brainstorming. What are your must-haves for your games?