*IMO* a bad idea. Asteroids are found in star systems, not in deep space (except in Trek War, of course

).
The problems that would arise would be, would you "colonize" the asteroid belt? Would you have (yet another...) ship type to mine those independent asteroids? How would you build structures on them? Could they be conquered or destroyed?
I'm not sure what this would add to the game. Systems already have Dilithium, Deuterium, Raw Materials. Nebulae will produce Deuterium. Asteroids in systems already are a source of more Raw Materials (pending a fix).
Just my opinion of course.