I'd be interested in something like that. Have played quite a few "Space Sim" (Homeworld, Haegemonia, Endless Space) games but they all end up seeming a bit shallow and are mainly RTS/Shooters. I realise that is due to programming and budget limitations and the companies need to return a buck. I would like to see something more like Settlers/Civilization with an open universe, maybe with alien races tossed in (along the lines proposed for Spore before it was dumbed down and killed off).
Anyway, I have been keeping an eye on
en.spaceengine.org/ which is an interesting Russian free space simulation software (SpaceEngine alpha 0.97 has been released).
While it is no way near what you described as your desire yet, it may someday be able to be used as the engine for such a thing.
Capabilities
All types of celestial objects are represented: galaxies, nebulae, stars and star clusters, planets and moons, comets and asteroids.
Known celestial objects are represented using data from the catalogs: galaxies (NGC/IC), stars (HIPPARCOS), star clusters, nebulae, planets (our Solar system data and known extrasolar planets).
Uncharted regions of space feature procedurally generated objects: galaxies, stars, star clusters, nebulae and planetary systems.
The observer is free to move around the universe. Transition between any celestial body and any scale occurs seamlessly.
The observer can move around in space using the WASD keys, like in First Person Shooters. Movement with inertia is made possible in Spacecraft mode or Aircraft mode.
"Select and fly" autopilot: just click on any object with the mouse and hit the 'G' key to automatically go directly to the object.
Automatic binding of the observer to the moving object and automatic selection of optimum flight speed.
Search for celestial objects by name. Search for stars in certain radius. Scalable map of the Universe and a planetary system browser. Saving locations and autopilot's journal.
Built-in Wiki system with descriptions of many known objects and ability to extend by user. Possibility of renaming and describing of any object, including procedural ones.
The orbital motion of planets and stars is calculated in real time, with the ability to accelerate, decelerate, or reverse the flow of time.
Showing of orbital paths of celestial objects, their rotation and motion vectors, labels and markers, celestial grid, current selection pointer and user velocity vector.
Volumetric 3D models of galaxies and nebulae with interstellar dust clouds.
3D landscapes of planets and stars: for many solar system bodies actual data from space probes is used, for uncharted and procedural planets the surface is generated procedurally on the GPU.
The exact model of the Earth's atmosphere (code by Eric Bruneton), adapted for the other planets.
Lighting effects: lens flares, solar eclipses, shadows of planetary rings. Celestial objects cast light and shadows onto each other.
Controllable space ships (alpha).
Localization of many languages with the ability to add new ones.
Ability to import users addons: models, catalogs, textures.