Infinite Universe with procedural generation.
To allow for a scalable number of players, the Universe should be infinitely vast and automatically generated as players enter unexplored sectors (yes, like Minecraft). Procedural generation of asteroids, resources, and other debris will have to be done for any new sector.
The djlib.spatial module currently has an implementation (albeit incomplete) of an ExpandingRectTree. This is a spatial data structure for efficiently storing a scalable amount of entities over a very large area. The added benefit over a normal RectTree, is that a new root node can be created to expand the current RectTree bounds to an ever increasing size.