I think hitting the four Xs seems like a good starting point:

1) eXplore, so you need some kind of ship analog that can move around and reveal areas, or at least go to them if there's no fog of war yet. Maybe you can see the places its going but going there reveals the name and some details.

2) eXpand, so you need some way to turn neutral systems into friendly systems. Could be a second type of ship, or something that happens automatically once you have enough population in a system, or whatever.

3) eXploit, so you need some kind of currency that the systems generate, which you then spend on more ships or upgrading your system infrastructure.

4) eXterminate, so you need a way to use your ships to attack enemy ships and systems. Could be another kind of ship again, but could also just require higher numbers of the base scout ship.

Once you have that working and it's fun to play, I think you'll be well on your way to making a great game. Just make sure it's fun before you start adding in more features. If the absolute basic gameplay isn't fun, then you've got a fundamental problem and need to re-think what your base gameplay is (for example, mario is fun with just a runner, pits, and platforms).