I really like the new Asteroid Fields, they not only add a dynamic to colony management, but they make the galaxy seem more realistic. But there's a couple of other astronomical entities that I think would also fit in well with the game.
The first would be Black Holes. No ships can travel within 1 parsec of one, they would have to go around. And of course no Stars/Planets etc. could exist within one parsec either. Two would probably be more realistic.
And then there's Nebula. They would affect movement in the same amount of space as a Black Hole, but wouldn't be quite as restrictive. Movement rates while travelling through them would be reduced by half and there would be a chance that ships suffer some damage while inside them. Ships with armour/shields etc. would stand a better chance of escaping the damage inflicted.
One thing I find lacking in the game, strategy wise, is choke points. Ships can go anywhere so it's impossible to really set up defensive positions. If there were some restrictions as to where ships could move, then proper placement of your fleets would become more of a consideration.