If you wanted to make it a RPG or MMO, you would need a server, and I don't think Dan can afford to buy and maintain a server alone.
And even if he could, I don't think it fits into the idea of Universe Sandbox - it's made for simulating, not for playing with other players. With lots of players being able to edit the same system, bodies could appear from nowhere or get deleted, and the simulation would become very strange with masses being added and removed. And what about the time step?
I like the idea of a spaceship simulator you could use to fly around with though, but that should be like the flight simulator in Google Earth where you play alone. Also, when getting close to objects, they might not be so amazing at all since their textures probably aren't more than max 2048^2 pixels.
I don't think it's a good idea to change Universe Sandbox into a MMO/RPG, since it wasn't what it was made for originally. If you want a space MMORPG,
Infinity: The Quest for Earth is in development and has a galaxy full of 400 billion stars you can interact with when it comes out.