Features to make space interesting
Asteroid, debris and mine fields. Besides other "units" these are the only physical objects you can interact with in space and the only detailed passive environment there. The only spaceborne equivalent to mountains, rivers, trees, silt traps and the many other features of terrestrial navigation.
They provide collision hazards (and victims thereof to be aided or preyed upon), cover from direct attack, cover from detection, mining and salvage, a credible destination (IOW not like the factories floating in deep, deep space) and scenery.
The three types are very similar in features but with some differences between them. Asteroid belts are randomly placed, static, permanent, enormous and sparse in both obstacles and resources. On the opposite end of the spectrum, mine fields are intentionally placed, dynamic, temporary and very dense (by design) obstacles and resources (at least from the player's mercenary and scavenger perspective). Debris fields are in between, they are the accidental side effect of intentional placements, like large stations, mining operations and battles, they come and go slowly, and are moderately dense with obstacles and valuables alike.
Nebulae are the closest thing to interesting or bad weather in space. While they are normally very sparse clouds of star stuff I *think* they become hotter and denser close to stars (the environment the game takes place in).
They obscure your detection ability and reduce its range and possibly accuracy. While they might be a source of reaction mass for your thrusters they also may inhibit the power of your spec drive. And since they span entire solar systems, only jump points can get you out of one (eventually). For the reasons listed above, nebulae are also destinations, even if only for those who want to seek stealth in space and those that are seeking them.
Nebulae would vary mostly by how many systems they encompass and how dense/hot they are.
Suns (ordinary), Magnetars and Black Holes.
Each of these influences the system around it, but at closer distances it becomes an environment in its own right, with all the dangers and treasures that naturally follows.
At medium range they offer energy, scientific opportunities and the occasional or regular hazardous solar flare. At close range, a slow build up of heat and damage, subsequent temporary escape from thinner skinned attackers and the assurance that enemies may only attack you from half as many directions, but the same goes for your routes of escape from said enemies. At very close range, a quick death.
Suns produce enormous light, which means an enormous source of energy and probably the only chance to produce any useful quantities of antimatter. They are also the least hazardous. Black holes produce no light, in fact they subtract it. This makes for an awesomely shocking change of aesthetics from normal systems. It also makes them only valuable for research and disposing of. . . anything. (Including those godly-powerful ancient-alien-technology anythings). Magnetar are similar, but maybe destructive at greater ranges and to a lesser extent. Both black holes and magnetars are very rare, no more than one of each in the game might be reasonable.
Ambushers Versus Pursuers
The environments listed above are good turf for ambushes, be they by a military, pirate, lunatic or personal enemy. You enter an asteroid field and suddenly multiple power ups and liftoffs are detected from nearby asteroids-- pirates were lying in wait. You are traveling through a dense nebula when suddenly a minefield appears ahead, you stop, but then aera fighters appear immediately at your flanks and interceptors cut you off from the nearest jump point.
On the other hand, if you are already being pursued, a more complex and obscuring environment might be your best chance at losing a more powerful attacker you can't outrun in open space.
But first, we need to have ambushes and pursuits in the game to begin with. In 0.5.0 getting surprise attacked or intercepted basically never happens. Which makes tactical use of the environments of space pointless. Bushwhacking and getting bushwhacked, intercepting and getting intercepted, should both happen and have interesting mechanics and strategies associated with them. Good navigation then becomes essential to not only dealing with real enemies, but accounting for the ever present possibility of enemies.
Space is big and old. Things are hidden in its vastness. Things are protected by the environments listed above. After much time spent exploring, simply encountering these "things" is pure magic. Without warning, seeing something rare and special float into view in the silent darkness. We need exploration to make a more engaging game and then we need treasures (rewards) to make more engaging exploration.
Unexplored solar systems, derelicts, salvageable scrap, mining and colonization opportunities, alien artifacts, undiscovered alien species, theoretical (or unimagined) anomalies. Features that fill out the game play of exploring and directly exploiting the resources of space (instead of playing star taxi).
You explore a previously uncharted system, and find within it a deuterium rich ocean world, titanium rich asteroid belt and an ancient alien satellite. You collect the satellite, sell it, use the money to buy colonization equipment and settle the ocean world, where you harvest and sell deuterium. Now you set up mines on the asteroid belt. Then in an adjacent system you discover a derelict scout ship from a yet unencountered alien species; you haul it back to your colony for repairs and give it to your wingman to pilot. Fast forward a year and the same new alien species is at war with the human confederation over territorial expansion rights. After a great battle between the two, you salvage millions of credits worth of scrap and technological secrets which you sell to both sides.
Inspection and/or repair of ship systems, both internally and EVA. Replace damaged modules with backup components, whilst following safety procedures and getting the job finished ASAP; the game does not stop while you are working and vulnerable.
Explore the competencies and emotional limitations of your individual crew members through social and practical interactions and assign them tasks on the ship accordingly.
Encounter friendly, neutral and hostile stowaways; simple lifeforms or cunning sentient species (humans, klkk, aera, etc.). They may turn out to be (assuming you hold your fire long enough to find out) anything from a nuisance to a real threat, to a potential recruit, to a plot fixer.
Fight off hostile boarders (or board hostile ships and stations.)
Use in flight time to handle longer term planning. Write logs and notes and draw up flight plans and backup strategies. Try to decipher codes, messages or devices you have intercepted or stolen.
FTL/long range communication
Sending/receiving SOS messages
If you are in trouble, you can call for help. Pirates just live for distressed fisshies to splash about too. Send one and help may arrive, but so too may more harm. If you pick one up you can choose to help or finish them off! Perhaps you can call for a tow if you run out of fuel?
Major events can't wait until you land for you to find out about them. If disaster strikes, or a system changes hands, there should be a report right way.
It may be possible to broker a better price if you have a buyer lined up rather than selling on the open market.