A return to Sol with a twist; there is no civil war in the GTVA. There is no massive, hostile power waiting in Sol.
Sol is mess of small warring factions. Earth and Mars are being held hostage by their orbital industries threatening to drop rocks on them, so the provide troops and material for war efforts they want no part of against basically everyone else. The Moon is divided between different factions whose only point of agreement is that if Earth tries to conquer them they put aside their differences and fight back. Jupiter/Saturn are an outsize military power because the relatively low gravity on many of their moons makes construction of spacecraft on the ground possible. The asteroid belt is a massive mess of small mining concerns who solve their differences with violence but if it ever looks like someone is going to assemble momentum and snowball then everyone gangs up on them.
The GTVA has to work out a way to clean this all up, preferably without a war of conquest. The player is tasked with what is basically spaceborne counterinsurgency at first, slowly escalating as the powers take sides reintegration, with a midpoint of the campaign being freeing Earth.
Then at about this point someone notices that Earth's second node works again as well, not just the one the portal fixed, and there's a general scramble to find out where it goes. Through it comes...the Ancients, who in a sublime twist of fate, mistake the GTVA for the Shivans and attack. Now you're fighting to defend Sol against a race that has every reason not to listen to you but whom you could theoretically make peace with while the various Solar powers continue to get up to shenanigans in the background culminating in Titan getting glassed by an Ancients juggernaut, the GTVA pulling out all the stops and deploying every major combat asset it has which scares the crap out of the remaining Solar powers to see so many destroyers since they don't normally build ships that big, and a push into Ancients space and finally a peace treaty when they realize you're not the Shivans.
This would probably take 50 or so missions, too long for an expansion, but is it what I'd like? Yes.