Its been asked for in the modding forum, but a map editor for GC2 would be nice. Now as to the idea part, also from the modding forum is the additon of more objects, of which some are impassable by ships. I personally think this is a great idea as it adds in the ability to have static closed boarders if done right, which leads to interesting tactical choices. Also you could have the barriers cuase damage per turn within their limits. As it is now I think there should be various types of these objects.
Solid types- Asteriods, detrius, solid hunks of matter etc. Unable to safely navigate these without a certain degree of armor, maybe specific to barrier. This would give added weight to the armor tech line since it would give greater ability to penetrate these areas safely.
Energy types- Ionic fields, Em fields, Nebulae w/ high elec currents etc. This tied to shield defense line and follows the above, better shields = better ability to enter safely.
Gaseous Types- this one could be tied to either solid, energy types or niether. Basically have this one limit sensor range by a certain amount, an improved fog basically. Also by tieing it to another type you could have a dust cloud with micro meteorites or static charges giving it the ability to be extra dangerous to expolre and guard.
Specials- Blackholes need i say more. Various sizes, never larger then a sector for the very largest. The hole it self is only a parsec or so but it affects ships by pulling them so many parsecs depending on size and or closeness. Would make for interesting escape routes/ entrance points for fast enough ships but not without danger since getting to close would mean being sucked in.
Also possiable stable wormholes would be a nice widget to add in, and maybe for building purposes the old time gateways. It never sat well with my mind for the races to abandon the gateway tech totally since its mainly a enegry problem associated with it. Since energy isnt a factor anymore, but the gates are still near instantenous travel why not keep them? It makes travel to far sectors so much faster.