#1 Mega structures
I especially like the idea of mega structures. Starbases are a great thing in this game, being able to add huge numbers of modules is wonderful. However, having grown up in the shadow of Star Wars, if I had a galactic civlization, I would totally start building planet sized space craft and etc.
Doesn't make for balanced game play tho--
so have them drifting through the galaxy, blowing up planets, and you can destroy them.
#2: Space travel makes the galaxy go round. Well, the galaxy got all squidgy and craft range is reduced by 95%, effectively landlocking spacecraft. Any colonies cut off from the others have lost contact, and become their own entities by the time the galaxy emerges from this.
Another variation on this is that "walls" tend to form between civlizations. So the event cuts off a large, dominant culture from conquest, while saving the smaller, weaker civilizations from certain doom.
#3: Moral decay
First it was the waltz. Then it was the bicycle. Now its cranial hyperwave penetrating gloviation. The older generation has finally had enough, and there is an old\young division in every empire in the galaxy.
Birth rates effectively stop for X years, or go negative as the old geezers off the young whippersnappers, upsetting the demographics of the empire.
#4 Black tuesday.
effects the dominant economy if it outstrips the others by too far a margin. Black tuesday hits. The markets have a collapse. Empires trading with this economy are hit badly as well. This event tends to greatly benefit evil, and more-isolationist empires. Those allied with the disaster economy have a hard time.
#5 Back to megastructures
The melenorme show up. They sell you an amazing space fleet, and a "dyson" style planet, 26 size, that orbits your home star, for some little critters on a moon of some planet under your influence.
Its free, basically.
This benefits the downtrodden empire with a fantastic defensive force and an awesome planet.
#6 Science is dangerous.
Anti-intellectualism becomes rampant in the most-advanced empire. The common folk rise up and destroy research infrastructure, smoke weed, and burn books. Some of the newest technology effectively dissapears.
Highly contagious movement.
#7 Its broke, and i can't fix it.
Corruption at the highest levels has given way to corruption at the lowest levels. The company contracteded to do general maitenence on your spacefleet was fakin' it. Flalse reports and etc. Your spacefleet effectively explodes, and the few remaining ships have one hp and need to limp back to port.
Effects the large military forces.
#8 Drug resistant oganism
Well, the quasiparrots of Vultar 6 are mighty beautiful, but the drengin got a little to personal with it and contracted Vultarial Mulbuttonism. Spreads rampantly through trade-connected populations, slower if not connected by trade.
Eliminates huge numbers of population.
More advanced socieities are affected more.
Sorry if some of this was posted already