The problem is, it needs to be really really well done.
For example even at its top SOE couldn't bring Coruscant to SWG. Because either people would be disappointed, Coruscant would lack stuff to do or it would be a massive lag fest. So SOE didn't even want to think about it for a second and they were right.
Ever played SWTOR? Nar Shadaa is actually well done but it is claustrophobic, full of corridors. Coruscant was like Citadel on ME, looked nice but so many transit hubs and "out skirts of town" had no real architectual connection to main area.(There was war okay but even ruins didn't look similar to starport places)
Talking about ME. Omega was well done but it was small. Citadel looked nice but felt like "corridors" in old FPS games.
So in short it is really a hard thing to do. I'm not sure anyone managed to do it yet.
A place full of life, also has "super tech post modernistic BS" stuff which actually is too "sterile" in most cases, filled with points of interests but has no stupid fetch quests, have no "corridor style" FPS map but also doesn't kill FPS and/or cause lag.
But if i was mister Hudson i'd aim for this instead of creating bazillion of planets. Because filling so many planets with something is yet another boring job. Fetch quests? People hate it. Empty areas? People hate it too. Nicely done side quests? Takes too much time to create and instead they could work on main quests.
Most people criticized first game for copy paste places, copy paste Thresher Maws, Mako handling, empty planets that have copy paste "dungeons" on it etc. And all these lead to "filling empty planets with something that can make sense".
Probably too late for ME:N but still my two cents.
PS: Even thinking and typing these made me tired, glad i don't work for world designer on Bioware 