It's obvious why Stardock has flatly declared ground fighting off limits for this game: in addition to the cost of tacking a whole new game onto the old one, it would upset the balance of the gameplay.

But here's an idea for adding some simple ground sims into those interstices of gameplay where there aren't enough resources to order up the next action and the player is reluctant to drive up black market prices:

Player zooms to the portion of the planet directly under a tactical defense unit, a trade port, or a broadcaster, and adds a fixture on the ground that will enhance the function of the orbiting unit. Once the fixture is added, some enhancement sims can be randomly generated around it over time. Another player trying to take over the planet later would see these sims when zooming in to tag the fixtures as bombing targets.

(Sims design shouldn't be too costly for the game developer: There are only four planet types and three factions. It would be the equivalent of designing a few new ship types.)

Rules for adding ground fixtures: They should not cost any resources, since the idea is to set them up when resources are low. But they should not be possible until both "civilian infrastructure" and "emergency facilities" are fully developed.

The player challenge would actually be at the point of placing the orbiting units: If they are not directly over an appropriate terrain, such as a land mass, there will be no possibility of adding an enhancement fixture later.

End result:

- Gameplay that actually requires the 3D navigational features that are already in place

- Something for the player to do when resources are low and the black market is volatile

- Extra eye candy and the sense that the vast battles in space correspond to assets on the ground.