People, people, people, it works like old russia, one palce to fly to on each planet, then move around to different areas in that one place, we havent even seen all of old russia yet we still have the bunker and kings watch, the graveyard and the dry sea to explore there, and the places underground blocked off by ??? level enemies, the places are big, its just you dont pick where you drop down specifically.
I doubt the places blocked by ?? enemies are as big as a lot of people think they are. You can run past the enemies in some of those areas and survive long enough to see that some of them are just "treasure rooms" with a chest or two and sometimes dead ghosts, etc. There are several places that are likely quite large side dungeons to explore though. Such as the Jovian Complex... which was just a dead end in the beta, as well as King's watch, which you mentioned. But those "underground areas blocked by ?? enemies"? Don't get your hopes up for them being expansive dungeons full of high-tier enemies to fight, adding hours of extra content.