To keep players from losing track of their ships I would make the camera follow the player. This way I can also spread out the playing field and delay the chaos a bit. I would also add asteroids, little mini objectives that give you more points, and interesting places to explore.
Background music is also on the list.
I think upgrades are a good idea, but im not sure exactly sure what upgrades to add, or how they would interact with the time travel re-spawn mechanic.