For first id like to have stronghold on icy planet of Ilum. it is really beautiful planet with very beautiful night sky with lot of stars. So what kind of stronghold would be down there suitable? well for first it should ahve good balcony at least or other area where to look nightsky while resting there. Maybe partially carved into ice pillar?

Player owned warship stronghold (kinda like guild one but smaller). I believe this era have many suitable warships to choose from. Maybe even multiple options from where to choose.

Guild could own multiple star ships. 1 flag ships and 2 auxiliary craft?

I don't have much experience about conquests of planets here but might be some ideas. Planetary assault is 2 staged process. first stage is spacecraft fight where aim for invader is to first clear space of opposing ships (for this purpose ships need to be armed by guild) and then make ground asasult. Maybe place for making more military strongholds for planets that guilds can own if they can defend them. For space fight both player ships and guild ships are fighting. the fight ends if either party withdraws or 1 party loses all ships (don't worry u get new ones at fleet but can't take part on ongoing fight). On ground assault both parties "amount of repsawns" could be tied to how much they have supplies crafted into reserve on that location.

Maybe 1 guild stronghold could be space station where ships could be docked.

All intention here is to generate stuff for people to do together and decorate/gear. SWTOR is at its best when doing things together with your guilds.