I've noticed that the idea of keeping Lelouch from giving Euphie the command to kill the Japanese is a pretty popular idea. Based on that, what if another major use of Geass was altered. That being Charles erasing Lelouch's memory after Suzaku caught Lelouch at Kaminejima. What if, rather than forgetting ever being a prince and being Zero, instead he just lost his memories of being Zero and became a prince again?
I would imagine that Lelouch could well be sent back to act as Governor of Area 11 and bait to lure C.C. out. Not to mention that, if there were corruption and any terrorism left, Lelouch would most likely grind it out of existence. Not to mention, imagine the reactions from Kallen, Shirley, Sayoko, and anyone else who knew Lelouch Lamperouge upon seeing Lelouch Vi Brittania.
I freely admit that part of this idea is based on a desire to see characters like Carline, Guinever, and the Knight of Twelve used more often. I can see any of them, or Anya, being sent with Lelouch to keep an eye on him... I also wouldn't object to Lelouch replacing Odysseus in Schneizel and the Eunuchs' political marriage.