The ideas posted so far fall into several different classes.
Things that could be implemented easily, because they're already "in-game":
- Comets.
- Rogue planets / dark systems.
- Making rapidly rotaing stars oblate.
- A means of obtaining some or all of the various currently-unobtainable permits.
Things that can be added only with great difficulty:
- Ceres, Phobos, Deimos, and other planets or moons in Sol or any other star system that are inside the orbit of the already-existing outermost planet.
- Accretion discs, protoplanetary discs, contact binaries that actually contact.
- Variable stars.
- Landing on Earth and other Earth-likes.
- Landing on Earth's Moon.
- PIlotable megaships.
- New star systems.
- Moving a star system to a different place in the galaxy.
Things that will likely never be added, for technical reasons
- Irreversible dynamic events (supernovas, colliding planets, etc)
- Large numbers of new stars (eg. adding globular clusters).
- Making the stars in the galaxy realisitcally move or orbit around the galactic centre.
- Other galaxies.
Things that will likely never be added, for copyright reasons:
- anything stolen directly from Star Trek, Babylon 5, Firefly or some other famous SF franchise.
Things which might have already been added, as far as we know:
- Rogue AI civilizations (human or alien)
- Non-Thargoid, non-Guardian aliens (alive or extinct).