I know this would be insanely hard to do, but a warhammer 40k mod would be cool. Imagine an Armageddon or Cadia scenario, or a random map where one faction starts off owning the planet and the other invades. Players can control members of a faction, like space marine chapters, imperial guard armies and the ad mech for the Imperium, chaos war bands, ork tribes, and tyrannid swarms all working for their larger race. Titans, orbital bombardments, shield generators, heavy infantry, deploying troops from space... There are no peace deals or capitulation, only the total annihilation of the other faction.
EDIT:
just to expand upon the idea, Instead of global tension you have orbital control or superiority. The defending faction starts off with 100% control and has about a week or so until the enemy fleet reaches the planet, at which point that score would plummet for the defending faction and sharply rise for the attacking one. The players can research on the focus tree to send out distress signals to gather more reinforcements from space, as well as summon new allies such as Space Marines and other Orks looking to join the Waaagh. These reinforcements can help shift orbital control when they get to the planet. Ships in orbit count as factories and can supply directly to star ports on the surface of the planet, which is important for attacking players.
It would be difficult to have space combat, but if there was a way to have another map mode that swapped from planet view to star system view you could take all of the naval combat mechanics and just stick it in there. I mean, just changing the map from earth is super ambitious and unlike 90% of the other mods, but having multiple maps sounds like a great technical challenge. If it could be done, that'd be so cool.
Eldar probably wouldn't use massive fleets but rather just pick hidden webways to spawn from. Similarly, Necrons could "awaken" from slumber beneath the planet. I'm not sure if Dark Eldar would fit this game (maybe as allies of the Eldar when defending a planet), but most other major factions definitely would. I could see this working for the Imperium, Chaos, Orks, Eldar, Necrons, Tyranids and Tau factions, each with separate smaller "nations" within them. Chaos can convert Imperium units to their side if there's a lack of commissar/inquisition research. If attacking, Tyranids and Chaos can gather intel about the enemy defending faction before they arrive and cause pockets of rebellion which represent cultists or genestealers, as well as using the already in place partisan mechanics to make regions unstable. I guess Tyranids can never be defenders since when they capture a planet it's not of much use to the other factions once they begin consuming all the biomass and then ditch it for the next planet.