Should incorporate ground combat, with the option of Auto battle, similar to Total War, and could use something like the TA Spring RTS engine, Dawn of War engine, or Supreme Commander FA, which work well with legacy systems, no need to re invent the wheel there. Have the option of 1 battle for the planet, or battles for several key areas or continents of the planet.
Turrets are a most, and maybe to offer something different more a modular system like the one used on earth 2150, with a basic frame and you add the weapons you desire.
Mecha units for space and ground battles, Merc hire system, Crime syndicates, Rogue AI (StarSiege, Terminator) with Uber mecha for space and land battles (Voltron) to spice things up, demon gate, dimensional gate, need to be destroyed before Invasion, also it could be a structure dropped on a planet or resource extractor that spawns units non stop until it is destroyed.
Asteroids and Battle Stations can be dropped on planets.
Options for normal or armed extractors. Units that excel in combat receive special weapon, armor, etc, player option to decide the upgrade. All units should be multiweapon systems.
Actually some modders are working on this, but it will take quite some time.
So dare to dream the impossible, as has happened to many old games that still are around, maybe before we are past our prime we get to play it.
Cheers and Happy 2014