Endless procedurally generated open-world omni-directional 2D shoot-em-up. Start with a weak spaceship and go on missions to find new weapons and ship improvements. Go to spaceports to buy and sell stuff, fight against aliens or Lovecraftian monsters, survive asteroid waves, find the treasure inside the hostile nebula.
There's pieces of that in Everspace, I could totally see your game working and being pretty fun in a similar manner, less arcadey than Everspace, but still accessible