The purpose of the blue/brown symbol is to let you know who is in your guild and who isn't...
Most people use the full guild names though, it's much easier and you see the other guild names instead of just a sea of brown circles.
Making a green one would just be confusing.

Player mods will never happen. People already let Star Guard rank go to their head...give them even more power and it's game over. Every day players can't be trusted with that much responsibility. They'll become nervous about losing their rank and become super-mods, answering every slightly off post with rule quotes and "please do not X" and policing blackstar. Eventually they'll get on everyone's nerves...and problems arise.