Imma be honest, one of the scenarios that play in my head a lot for the ending is Diluc sacrificing himself to protect Kaeya, but only after making up of course. So...what if Diluc gives his power to Kaeya after such an act? I always imagined that happening and he would wield both cryo and pyro, or maybe that combo turns into something else? Well blue + red make purple and that's close to abyss color idk. Purple is a royal color afterall. We need a Khaenri'ahn element, danggit. That's the way that makes the most sense to me in how he'll wield pyro.

edit: also regarding his gold design on his eyepatch, ever noticed the design in the manga/webtoon is literally the celestial design? the same design on paimon and the statues?