If the space battles live to their expectation, why not expand the concept to ground battles ? I really like simultaneous decisions (anyone tried Frozen Synapse ? It's a game in itself, so ES can't really borrow the design, but it can show how good a simultaneous combat system can be)

Yeah, I agree, it shouldn't be too intricate. But TBS could handle it aswell. Only problem I would see is that it would take potentially long times (hence optional) and not sure about RNG scenarios for those.

The best way to implement Ground Combat IMO is neither to expand the game with an RTS section nor a TBS war game. Just as the space battles are in ES, the ground combat should be resolved with a mini-game which faces different types of units against each other; in a "civilization like" unit combat scenario. Each unit can have specific abilities and be upgraded through technology research, but there would be no unit customization; to keep things simple. I had envision a system like this for a game I was working on, now with ES on the horizon, I can only suggest a similar feature for the game.