Did anyone play Rules of Engagement and Breach in the 90s? ROE was ship to ship combat, and you could board enemy ships. Breach was shipboard combat game that allowed you to take tactical control of those battles. ROE would just move you into Breach to play out the combat.
What about an optional planetary combat system, provided as DLC? It wouldn't unreasonable to charge for it. The hex maps from GC3 really laid the groundwork. Not to mention whole new tech trees for infantry, mecha, artillery. Maybe even be able to design your ground units?
A tactical control space battle system would be an option to. I'd like the idea of being able to design ships that I would carry out a specific role in a battle.
I had a dream for GC3, I am bringing it to GC4! LOL