Well, I kinda want Lelouch to know C.C. Not sure about Geass yet; that can be your call. But if he never got sent away when he was young, and C.C.'s always been stalking him, as canon indicated about his summer in Japan maybe C.C. and Lelouch met when Lelouch was still a child in this verse. Not sure if Nunnally knows her but probably?

Unless you had a particular AU idea already in mind.