Galactic Civilizations III.
-Availability of really huge maps, with up to over a dozen of simultaneous opponents.
-Over a dozen of different races/civilizations to play as, or have as opponents, with different traits, skills, and tech trees.
-Custom ship designer.
-Expansion and Preparation (and to some extent, diplomacy/relationship management) are the most important phases.
It's turn based, but it seems to fill all your requirements. And it's awesome.