This is inspired by a recent post by Incarnate.
Player or guild owned stations, of course, can only be built in grey space. I suggest that to implement the building of a station, a mission could be available at the nearest faction owned station. The construction need not use any new components other than what is already out there, just much more of them. Once the mission is taken, an exorbitant amount of raw materials like carbonic and ferric ore, and an equally large amount of trade goods like outer hull plates and data link connectors must be hauled to the "parent" station. Perhaps extra procurement and mining missions could be made available for guildmates of the party that took the mission. Stations could only be build able in empty or unmarked roid sectors, also only visible to the guild and other keyed parties.
As far as defence and capture, I think it could be similar to the current conquerable stations, only harder by about three times. More turrets and more NPC guards. The station could cost a weekly fee, drawn from the guild bank, and all storage be accessible to owner key holders. The station would not show up on galaxy maps and if unkeyed, you would not see the dock symbol on the radar, much like the conquerable stations.
If for some reason the station dues are not paid, the station becomes unaligned and hostile to all who enter the sector until it is conquered by another group.
Again conquering these should be really hard, certainly not soloable.

+1 googl.
That is, +10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000