"Presenting! Zynth's idea to revolutionize Gallia and make Discovery fun!"
Now with less systems.
Gallia and Nomads somehow attack at once, forming a pincer movement on the whole of Sirius.
Bretonia falls to the Gallic invasion. Rheinland falls into Nomad control. Whatever is left of the Rheinlanders sign a truce with Liberty, effectively ending the war. The remaining Rheinlanders flee to Liberty and help them against both the Nomads and the Gallics.
The Nomads are no longer friendly to the Outcasts, the Nomads plan a full scale assault and started infecting Outcasts in order to aid their plans to wipe out mankind. The Corsairs and Outcasts flee their systems after streams of Nomads come through the holes. The meet in Theta, putting aside their differences to fight off the aliens. They meet up with the surviving Rheinlanders and helped them secure an escape route to Liberty. They end up in Liberty, fighting against the Nomads on the South Eastern front.
The SCRA's home system has also been overrun from a Nomad fleet coming in front the South-East of Sirius. SCRA puts up a fight but had to retreat. They seek refuge with their now allies, the Colonials in Coronado.
Liberty is hit on both sides, the Nomads through Alaska and Rheinland and the GRN + Kusari team on two other fronts. As the fighting escalates, the Nomads and Gallics have first contact while fighting in Liberty.
The Gallics realize the threat to humanity after certain catastrophic events that led to the destruction of half their fleet due to the arrival of a Nomad Sun-killer (as seen in the extended OP). They sign a truce with Liberty and helps them fight the Nomads. The Order makes a comeback, securing the escape route to the Coronado system for the stranded Gallic - Kusari, Liberty - Rheinland and Corsair - Maltese fleets. They retreat to the Coronado system and meet up with the combined Colonial - SCRA fleet.
In the Coronado system, was the final stand. Humanity stood as one, for survival. It was do or die, eliminate the Nomad invaders or be vanquished as a species.
Plot good enough?
Naturally, the Nomads will wipe out entire systems and destabilize a hole here and there. It is an inRP excuse to remove entire useless systems from the Sirius map.
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Ok noted, the above is highly..um...improbable. But yeah, more Gallia interaction with Sirius. Even if it is through pew. Always remember, the closer you are to New York, the more activity there will be.