Something along the lines of Borderlands.
But you've got multiple planets.
Every planet is procedurally generated with a procedurally generated questline (as in series of quests, not just a bunch of random kill x monster crap).
Also, there are varying tech levels, so one planet might be full of energy weapons and the next has nothin' but swords.
And with more available options for the procedurally generated weapons than Borderlands.
Instead of 6 options for 6 parts, 20 options for the main parts and however many accessories you can cram onto it.
Also, incredibly in-depth character creator, also with a random setting so you can have procedurally generated characters.
And the characters have procedurally generated skill sets based on a set of given preferences.
If you say you like sneaking up on people, it's more likely to generate skills based on stealth.
You start out on one of the level IV planets (tech level four, that means spaceships and ballistics).
This one is less procedurally generated than the others.
You start out as a nobody with a gun and decide to make a name for yourself.
It's your choice whether that's mercenary, law enforcement, pilot, whatever.
Either way, you end up helping one of the two factions on the planet, either the government or the pirates, and eventually get a simple spaceship that you use to fly around the procedurally generated universe.
Factions are also procedurally generated, but the starting planet always has the government and pirates.
Think of the possibilities...
You could spend your whole life playing this thing.
Also, you can co-op with a small group of people.
There's also modding, but everybody who's co-oping with you needs to have the same stuff.
Say you create an accessory for your gun that projects a small forcefield, then your buddies need to have that also, otherwise the game will crash.
And you can craft stuff too.
Say you loot this gun with a really nice scope and a long barrel, but sucky everything else.
You can dismantle it and rebuild it with better parts.
Repairs too.
Gotta keep that equipment in top shape if you don't want to die.