Try to create a Code Geass fanfiction where instead of receiving Geass from C.C. Lelouch gains a Solar Exaltation. You can decide what caste he is in and what he does with his new found abilities after that.
Also you may decide to add other Exaltations if you want. But Geass still has to be a power and Ragnarok still has to be Emperor Charles endgame, other than that though, go wild.