After Lelouch is banished from Britannia, Cornelia, blaming herself for Mairanna's death, forfeits her royal standing and goes with him and Nunally. Euphemia would be optional, or planne to join them, but then the war happened.
Crack Idea:
After Euphies's announcment for the Special Japan Zone, Milly throws a party, and Lelouch gets drunk, brooding over the issue. One things leads to another, and he ends up sleeping with Milly/Nina/Shirley. After the rebellion and the memory erasure, life continues on, but now Lulu has to deal with a baby. It get worse when he regains his memories and he has to juggle destroying one o fthe world's uperpowers with not just school, but changing daipers and feeding his loyal minion to be.
Alternatively, the memories of the party could be among those erased, so now everyone's freaking out about the immaculate conception of the class baby. It would probably be best for Shirley to be the partner in this case, since Milly would most likely use the opportunity to force lulu to marry her.
Edit: More Crack:
Lelouch challenges the Emperor's reign through a Revolution.
I mean... DANCE DANCE REVOLUTION!!!!
Yeah, watched Robot CHicken.