lore-wise I think that when the TEC was formed out of all those trader worlds, simply not all of them joined, so some stayed neutral. those are the planets you have to conquer all the time. and some of them happen to be more developed and thus are colonised and have hangars and more.
I would like some extra emphasis on those neutral worlds, like that they develop on their own to a certain degree. and a non-bloody solution to get them into your empire, or diplomacy with the in general. like the minor worlds in gal civ II. and of course I'd like an option for them to be in the standard maps the normal random maps. make it a slider or an 'on/ off' switch if you want, but I'd like them.
I would suppose it would be second mini-exp content, but tight lipped as the devs are, it just that. an assumption.