J_Titan
If u can't think of any and can't fine any good inspiration... How about making some spiff off of Einar in "Fate Grand Carnival" it doesn't need to be a serious story just some funny daily life he have in chaldea with a occasional flashbacks on what happened to him and some characters after the whole Camelot stuff?
Tbh I just want side stories to know more about the future of Einar with Scathach / Mordred / Morgan / Altria / Goddess Rhongyominiad (cuz why not) / Alaya An idea is just Einar traveling across the nasuverse with each travel being a different altria or something like that Cuz I do agree that DxD is overused and is way too different compared to FGO, but you could make it well pretty fine I think