There's lots of actual stuff in interplanetary space that you can pull on for inspiration on how to make an interesting game.

You can have counters with shady trader types that are only in the vast gulf between the systems, there could be rogue planets with billion year old abandoned cities to explore filled with automated defences for you to fight and interesting loot at the end. Distant ancient asteroids that contain the seeds of the first life in the universe that when you interact with temporarily give you status change that you can only get from asteroids and temporarily gives you super strength or something, allowing you to complete missions in a way you otherwise would not necessarily have done.

The way these kind of side quests are supposed to work is the player is plodding along trying to get from point A to point B and on the way they get sidetracked by this side quest (the clue is in the name Bethesda). Maybe it changes their priorities or how they're going to tackle and upcoming mission. Side quests are not supposed to be independent standalone things, they're supposed to integrate with the main story. They're not supposed to be something you find easily there's supposed to be something you come across on your own as you're exploring the environment, but you can only do that if the developers bothered to provided environment for you to explore. If they just teleport you to your destination then there's no opportunity for this kind of emergent gameplay.

Loads of stuff you can put between the star systems.