It would make an interesting 'parallel' storyline. Let's go back 8000 years: the Shivans are pawning the Ancients. They're fighting in the nebula at the moment. Desperate to save their lives, the Ancients finally turn off the Knossos in GDrax (Why did they wait so long? Perhaps special resources were needed, or they had to do it between the Shivan attacks, I don't know...). However, a tiny part of the Shivan task force has already got through, and that was enough to destroy the Ancients. Cut off from the rest of their race, these Shivans float around for 8000 years, until they detect intense subspace usage again. That small part of the old Shivan fleet is now known as the Lucifer fleet.
During the Great War, a part of the Terran fleet gets cut off from the rest as well (the majority of our fleet was in Sol, it was about the only system the Lucifer did not sweep clean). The Shivan cut-off part gets destroyed.
In the meantime, something is scaring the Shivan main fleet quite a lot. They build Sathanas transports, designed to evacuate all Shivans to some far-off place. But they can't find a star meeting their needs. When Bosh re-activates the Knossos in GDrax, a Shivan recon cruiser comes through, sees that GDrax doesn't meet their needs, and proceeds to Capella. There, it encounters the GTC Vigilant, and destroys it (they're xenocidal, after all). Before it gets destroyed by the Chartage and the Dahshor, it 'scans' the Capella star, and sends a message to the other Shivans: 'This is the star we've been looking for'. Immediately, the Saths converge on GTVA space (one was probably stationed closer than the others), while the bulk of the Shivan fleet -which could be manned by robots- remains behind to buy them time. Only a few ships escort the Saths to Capella. The Saths jump to some far-off place, closing the door behind them. The other Shivan ships, unable to use the supernode, remain behind to try and eliminate all witnesses. During the Second Shivan Incursion, a significant chunk of the GTVA fleet (the cut-off part) gets destroyed as well.
Dare I say it - FS3 would hardly involve any Shivans: they're destroyed by the Capella explosion and/or by that which they were running from. (We would have re-opened the Sol-DS node in between.) Instead, we would meet this huge enemy even the Shivans couldn't defeat, and they would chase us all over known space, just as they had done with the Shivans. The Vasudans probably get completely annihilated. GTVA Command is desperate. All humans retreat to Sol, closing the door behind them (=blowing up the node).
The trilogy would end as the story once started: Earth will always remain isolated.
