Nation of Immigrants: Have your starting species stop being your loyalist faction.
Remember the Ancients: Have not a single member of your starting species exist in your empire, but maintain the same ethos
One of Us! One of Us!: Have a foreign species pop be completely converted to your starting ethos
Don't Change Your Horses: In a democracy, have your current ruler be elected three (maybe 6?) times in a row
Hello, Boys, I'm Back: Have a leader be in power, lose the election, and then be elected again.
Istanbul was Constantinople: Conquer a foreign homeworld and rename it.
No Home to Go Back to: Have an independence faction succeed and take your homeworld with it.
Send in the Janissaries: Using land units of a foreign species, use them to attack and conquer their Empire of origin.