I vote for Jupiter and it's moon system. There are many moons to explore there. The UI interface can work in a similar fashion to the reef. By selecting it, you are moved to a "Sub system" where you can select a moon to explore. I think this area would be a great venue for the Cabal. It fits their style. Gigantic planet, crushing gravity, many many moons as outposts that can see a lot of different angles all at the same time, and not to mention the extremely dangerous radiation belts when approaching Jupiter. It would make a great planetary fortress of sorts.