Here are some ideas you can use if you want, not sure if any of them are very good
For spaceships, combat, exploration:
Players would choose a combat, scout, and transport ship to colonize the local group and fight battles and junk
Planet colonizing:
Players start out on a home world to build their empire from; players expand their borders across planets and star systems instead of countries and continents, with different techs needed for extreme planets, gas giants, neutron stars, nebulae, distant systems, etc.
War:
When players wage space war, they fight in space or capture planets; capturing a home world usually ends in whoever originally owned that planet being wiped out
