I would go with:
Freelancer: spacesim which has a sandbox of a universe
Deus Ex: Complex FPS/rpg, we all know
Homeworld: space RTS
The game would have you as a person (1st or 3rd) and you can fly around in your ship through space, land at stations, walk around and do missions a la deus ex. But also in fly in space exploring. and when you work your way up in the game you can get complete control over massive fleets/ships and walk around in them and use them to conquer planets. Conquering planets would have you use your fleets to secure the system and then set you down on the planet to lead your space marines into taking over.