Kerbal Space Program meets Sim City meets EU3 meets Star Control 2 meets Sim Ant meets Sid Meier's Alpha Centauri meets Scorched Earth meets Homeworld
1. You would be in charge of a civilization and more importantly that civilization's space program
2. The playing field would be a portion of a galaxy with several star systems, including all planetary bodies, comets, etc. found within, as well as those objects as playing surfaces themselves. If you've played Sim Ant, imagine the yard overview screen. That concept, in 3D, in space, without ants.
3. Game mechanics would involve you building rockets, assembling space stations (using the rockets), landers, moon bases, other planet bases, and just generally attempting to spread your influence first through your own solar system, and then later to other solar systems. Imagine a simplified Kerbal space program interface for parts of this, then a city building interface for your bases, once they're advanced enough.
4. Claiming a planetary body would depend on its size and whatever else and could include several EU3-like stages from "not yours" to "core", depending on how big your base is, what sort of stuff you've built, etc.
5. Terraforming a la SMAC would be a part of the game, and there could be also different species with different atmosphere requirements. I see this as a very interesting potential game dynamic.
6. When two warring civilizations end up on the same planetary body, they can fire stuff at eachother a la Scorched Earth or angry birds.
7. Planetary bombardment and space battles a la Homeworld would definitely be in there.
8. A simplified Sim City like base building/management system for your bases might be overkill, but you would definitely want to somehow eventually build factories, habitats, terraforming vehicles & modules, etc. So it might as well be fun and done in a tried and tested format.
9. Diplomacy and trade between players should be possible in some way that doesn't suck.
10. The Star Control 2 element would be a story that evolves as you play the game in single player mode. Like in SMAC
I would call this game
Awesome