Writing Good Adventures
Summing up so far.
Know the Ending.
Work small plotlines, gimmicks, pushes, pulls, and enigmas in among the bigger ones.
Theme and environment, in broad sketches, for places and scenes and props. Break cliches. Worry about the story more than creatures.
NPCs. History, family, personality, blind spots, plans for the future. Break stereotypes.
Example
I want the final scene to be a battle between the players and a main Bad Guy and his henchmen, aboard the Bad Guy's spacecraft, in orbit.
The bad guy's starship should have a memorable theme, then. For example, it may resemble the clean, dark, technical angularity of the Death Star, with a generous "boardroom", wide hexagonal corridors, dangerously heavy gravitically-controlled fire doors, and dual interfaces at workstations - one for sophonts, and one for robots. That last bit provides a hook for a robot player (there's a potential gimmick) or an NPC robot owned by a PC (there's another potential gimmick).
Perhaps their main goal is to steal the Bad Guy's Black Globe Generator. That's a cliche taken from Star Trek, and I have to break cliches, so maybe their main goal would be to swap the BGG with a non-working copy (or, perhaps, a fancy bomb that slags the engine room). So what's the push or pull behind that? The Bad Guy and his ship must be a threat to the players somehow. If they players have their own little outpost or base, then the Bad Guy may be en route to destroy it -- but that cliche is taken from Star Wars, so I have to break it, so maybe the Bad Guy is out to capture the players' little base. So that's a push, right? The Bad Guy pushes, so they have to neutralize the Bad Guy in order to defend themselves.
That means the players' base is a gimmick as well. It must be something special: assume the players themselves haven't put time into customizing their very own base. That means the base itself is valuable. It may contain something special, such as a non-working pre-Maghiz Darrian relic starship which is repairable and has some impressive features, and the players can potentially obtain the missing parts to get it working again. This may also mean that the little base itself might be TL16, though perhaps only partly functional. So the base has its own theme - exotic, old, and of undiscovered potential.
So the Bad Guy's BGG is also a pull. Maybe components of the BGG can repair or replace something broken in the relic starship.
The point though is to give the base value to the players to use; the players could also sell the base and its craft, in which case the plot becomes complicated by having to line up prospective buyers and at the same time staving off the Bad Guy... who in fact may have originally posed as a potential buyer who just wanted the thing for free, to further his own personal reign of anonymous terror. Or perhaps he's a powerful factor of Arkesh Spacers et al. So his first attempt is to scout out the base and try to take it via henchmen. (So it turns out that this thing writes itself.) There's the manifestation of the push.
Thus we have a few major NPCs already who require histories and personalities, including of course the Main Bad Guy in this segment, but potentially also the "NPC robot" if there's no robot PC, because an NPC robot is not just an automaton, but will in fact have a history, potentially could even have a "family", and will have blind spots and "personal" goals. It may or may not be a full crew member; if it's a possession, there may well be questions raised about slavery by the players; let them mull that over. At any rate, an NPC robot will require compensation in order to work toward its own goals, but the nature of that compensation depends on the NPC itself.
So anyway, the players need to get aboard an enemy vessel. That means smuggling themselves aboard as cargo; or being captured and having a confederate aboard who can free them; or impersonating crew members (if the ship is large) or being hired as new crew. Typical Traveller fare.
To be hired as new crew, they would have to infiltrate the sort of sordid company that said Evil Bad Guy uses to conduct business, and build some sort of trust there. So they have to act sort of evil for a bit, which could be an interesting masquerade. This means there's a pirate base/freeport out there where the unsavory, the unwanted, and the outcast hang out. They're not all evil, either -- every society rejects some people based on non-evil premises, and those people are forced to mix with true evil. Anyway, the base will have a theme, probably a measure of neglect and dissipation along with unreliable, unsafe "modifications" of equipment and ships. Could be a marketplace for special items, including drugs and slaves and weapons. Potential gimmicks there.
Now, how do the players learn about the BGG on the Bad Guy's ship? Well they have to overhear a discussion or intercept a transmission that ordinary law enforcement personnel would not be able to detect. Maybe this is auxiliary to their trying to neutralize the Bad Guy? So they already decide to stop this evil guy, and only after they've spent time in the pirate base/freeport do they hear rumors about the Bad Guy's BGG. So that's when the pull comes in, perhaps.
Winding back, then, the earliest scene so far is the players' discovery of the small base. The referee can just give it to them at the beginning, or it can become a scene in and of itself in order to build its value in the players' minds. So they're doing some horribly boring job for someone, prospecting in some asteroid belt, when they stumble upon the base. They get to figure out the puzzle of how to reactivate the power source, then how to send the "open hangar bay" command, then they explore it.
It has to be off the beaten path; otherwise some government would claim eminent domain or immediately send a flight of patrol cruisers and strike a "bargain" with the players. This requires some thought. The base can't be so powerful that it poses a threat to anyone, nor should it be useless to the players. In other words, great for players and pirates, generally useless for strategy.