The point of this is to talk about difference game ending scenarios.
One of mine would be a biological species like the tyrannids that come from the darkness between galaxies. They would gain the features of the creatures that they kill. If they invade one of your planets they will gain your species traits and make creature that directly counter your troops on the planet. This would make stopping from from reaching planets be your primary concern. One important part about game ending events is overwhelming force, this would need multiple EXTREMELY powerful fleets to spawn around the edges of the galaxy. This would make it impossible for you to single handily take them on, you would have to work with the other empires in the galaxy to defeat them or let them devour your enemies and clean up what’s left of the invaders. Maybe you could wait them out, their “ships” are biological creatures that will starve if they don’t feed on life from planets.