Something a little more epic.
After conquering Earth, your civilization has to settle in the solar system, setting up cities on a terrformed Mars, Europa etc. After that, you need to conquer the galaxy. They could easily add billions of star systems for you to conquer against thousands of alien races.
The current base game set on Earth could now just be the introduction.
With this, they could turn the playtime from hundreds of hours to years before you could complete it.
As well as this, add real time battles like the Total War series.
It would be the most epic game of all time.