I think a no-war AU with Kokichi being Eito's older brother is a golden concept.
Maintain Eito's horrible brain disease, but make it so that Kokichi is the one person who doesn't look like a demon to Eito's eyes aside from himself, and his whole concept of him being sent by God to judge mankind falls apart. His parents (Nagito and Mikan) are so nice to him, so patient, so caring, so understanding no matter how scary they are and Kokichi, his other frame of reference for a human being that looks just like him, is a complete piece of shit.
I think it's also a golden idea because I like imagining complicated sibling dynamics. They're both different flavors of "stupid chud son with psychopath tendencies", and they get into so many little petty squabbles it's crazy, but they both truly love one another and look out for eachother. More often than not, saving eachother from themselves.
Kokichi lies for Eito and perfects his craft of being a manipulative liar by defending his little bro whenever he gets caught out for even the most vile and blatant acts while Eito becomes a master of intrigue and scheme crafting by shadowing Kokichi and improving his older brother's more boisterous and showy plots, stopping them from blowing up in his face. And while in no uncertain terms, they are assisting eachother in acts of evil and mischief, in far kinder words one can boil their relationship into two simple sentences:
Kokichi will always be there for Eito.
Eito is the one person who truly knows Kokichi.
And I think that'd be a pretty fun idea for a comic series.
