We need some sort of purpose in our game. A narrative or goal (or both). What do we want our historical sci-fi to be like? Let's brainstorm! Here's one idea off the top of my head.
Essentially, take an aspect of the Roman empire and replace everything with sci-fi stuff. Instead of chariots, we have spaceships. Instead of centurians, we have...some hardcore marine-esk task force. I don't know, but basically a reskin of what actually happened with more high-tech stuff.
That's me without thinking, so I bet you all have much better ideas. Share your own and let's build off others!