You delivered Yavin stronghold during SOR, since you had assets to easily use, but there was no real story associated with its take over and it came very far after SOR was introduced. But it was at least tied to an active area for dailies and the entrance to an operation.

Manaan was tossed in due to popular demand. I have no interest in going there since there is no active (and very little overall in the game) story in Manaan.

The Umbara train has potential, assuming I head cannon stealing another or something. But I fear we will not spend any more time on Umbara to justify actually establishing a stronghold there. Not just for some crystals.

Iokath would be cool, assuming the story returns and includes a significant stay. Or wait until wherever the story goes next. Add a side quest with a complex, repeatable, but solo-able boss (even though I'm an operations guy) then we can "move in" (after killing him a few times).