Come on game industry. Let's get back to basics. A game just set in our solar system with colonised planets and moons would still be absolutely massive.
Kerbal Space Program.
OK, it's not real scale (mainly for gameplay reasons), but can be made so with mods. What's interesting, it didn't start with a huge idea. It started as a very rudimentary physics sim that simply kept growing. Sure, it has its flaws and the graphics are medicore at best, but compared to how things look for other projects operating on such a scale, well...
Another thing to note that it has never involved any sort of multiplayer. Sure, the idea has been floating about, but it's always been a sideshow at most. So yeah. Start small, work your way up and stay away from grand multiplayer schemes (at least until the point you
really don't have anything better to work on). To think of it, FSO (and many other open-source projects) also worked out that way. On the other hand, when ArmA3's devs tried focusing on multiplayer in the latest expansion, it was a complete disaster. Make of that what you will.