Simulation design, in this case. This is a strategy game set inside a simulation game, and that's pretty unique. The simulation game has to be robust enough to be interesting and create great stories and situations, but at the same time it has to be streamlined enough to be digestible.
There's also the whole thing of making it so that so many more different things can happen. It's kind of like level design, in that respect. Designing out more and more events that can happen, more contracts you can take, more ways to interact with the races, more ways they can interact with one another, etc.
For instance, today I was working on a new and improved way of smuggling, which involves blockade running rather than just normal combat. Then I've been working on having racial minorities on planets that they don't control, and a model for them that includes expatriates, refugees, prisoners, resistance fighters, and ground troops. It makes certain parts of the ground combat simulation more realistic than it was before, and also makes it so that every time the last planet of a race is captured, that race isn't out of the game permanently (pirates aside).
Then of course that opens up new doors with actions that the host and minority races can take against one another, events that might happen there, and so forth and so on. Meanwhile Josh has been working on designing more and more planet-level military, commercial, residential, industrial, soci-political, medical, and environmental events and their effects, then tying those into actions that the races can take or contracts the player can take. As well as coming up with thematic variants of common ideas to fit the various races (for instance, some things don't apply to a race that is a hivemind, but some variant of it can work).
Etc.
You wouldn't believe how many documents and spreadsheets we have for this game's simulation. A lot of it is stuff that won't come up in every playthrough, and which isn't something you have to pay close attention to in every playthrough even if it does. But it's all stuff that could turn out to be critical in some game or other, and it's all those nuances that will make it feel like a living solar system. But again, making it so that's not just a giant spreadsheet of overwhelmingness is where the trickiest parts come in. So there's a lot of GUI design and redesign and so forth that is going on for the subscreens for all that sort of thing.