When I got Mercenaries, one thing I was interested in seeing were the new minor races that would use the new shipsets. This because minor races is something you will deal with very often, and unfortunatly, it doesn't take long before you've seen them all. This makes the universe feel less exciting as you always see the same few less ambitious guys hanging around, while due to the faction editor you can have an extreme diversity in major factions.
Currently, a good solution to this is mods, but mods disallow multiplayer. I think that just adding a few more for flavor wouldn't exactly be hard. I am sure you got some scrapped alien 3d models hanging around that you can insert as new minors, perhaps offshoots of existing ones if there are scrapped Drengin, etc.