I had a suggestion about how asteroids could be used.  In GalCiv II they were used for production points when upgraded.  How about bringing that back but having the asteroids have a limited amount of mining points before they just disappear and can no longer be there to be mined?  

A planets attraction and how much emphasis is placed on building up that particular planet could be greatly improved by mining focus.  Would be great for early game build up for an economic empire and late game on outlying planets that have not yet been mined.  You could create a big burst of production points but for only a limited time.  This limited resource seems to be available in many strategy games (forests in Civ, Settlers series and such)