Damn, a quick google test seems to indicate that there's already a game called "Fallen Imperium".
Anyway, as promised, my idea for a space opera setting:
(TL:DR for history buffs
below 
)
Imagine a galactic empire that has overstretched itself, initially dominating other races, but gradually incorporating moderately powerful races to bolster the borders, creating a stable Imperium.
But the Imperials are being weakened by internal struggles for power.
The vast borders are being pressed from all directions by "barbaric" races who covet the Imperium's wealth.
The Imperium is increasingly dependent on the Border Races and Auxiliaries recruited from the barbarians - and the Border Races and Auxiliaries know it.
Now a new Empire is rising, and each new race it absorbs increases it's strength. It hurls itself against the Imperium.
Each battle shows the dependence of the Imperials, who continue to consider themselves to be superior to those they depend on.
The Auxiliaries rebel, and the Imperium is forced to concede territory to placate them, while the Border Races become increasingly independent of the empire.
The Imperium forms a coalition, with the Border Races and the former Auxiliaries, now as allies rather than vassals, which decisively defeats the Empire in a massive battle.
Soon the Empire's vassals rebel, gaining independence.
With the Empire weak and struggling against it's former vassals, the coalition breaks up.
Now it's anybody's game.
Short version for history buffs:
1. Take:
- The decline and fall of the Roman Empire (particlularly the Western Empire).
- The rise and very abrupt fall of the Hunnic Empire.
- The gradual shift in Imperial power from the Romans to their former Auxillaries (particularly the Goths).
- The eventual shift of the very title of the Empire to another people, as with the Frankish "Holy Roman Empire"
2. Add Laz0rz.
3. Profit. War and Peace complete (actually War and More War, which is better).
Time for beer & tanks.
Ti 3403
