Lelouch is one of my all-time favorite MCs, and there are so few long crossover fics. Here is an Idea I came up with in math class today. NOTE: I ship LelouchXC.C.. If you don't, just ignore everything I say about her.
Start after Zero Requiem. Lelouch is sent on a journey instead of staying dead. Have him visit the world of DxD and get magic. (I suggest something simple like Fire, Ice and/or Water, Air, ect. Something simple that can be adapted to the new worlds after this).
After he finishes the DxD trilogy, send him somewhere else. I suggest :
• The Familiar of Zero
• One or more of the Fire Emblem series.
• Sekirei
• Fate/Stay Night
Or anywhere that doesn't require him to fight alot. He is a thinker, not a warrior after all. I also suggest having C.C. talk to him telepathically.
When you are ready to be done with this -After you finish the world your on of course- have him pulled back to C's world, and rewarded. (I suggest reuniting him with C.C., getting a code, and the power to travel between which ever worlds you sent him to at will, and take C.C. With him).