People have been talking bout fleet starbases for a long time, and i to wish to see something you can do as a fleet. but instead of having a starbase which has limited usefulness ... you could have a few known planets that you can colonise with your fleet, think of it as a small town (that can get bigger if you have enough people) where you can modify your defenses and can have stores for everything (with a little group effort)
in my mind this planet would be one huge map, and not a whole lot of instanced towns. that way you could attack other fleets, or trade etc. The point is ... you'd have more options to get every member of your fleet useful instead of just the fleet leader adding things to your starbase. people could own a shop etc. Express themselves by giving their whole town a certain style, colours, streets etc. Just an idea, feel free to brainstorm.