I have seen this done a few times before but they are either poorly writen, have not updated in a long time, or they are suddenly droped. Any way I would have Emiya born 4-6 years before issei, this way he can have time to be established with his magic in the world of dxd.
He would also need to have a job in the town of kouh, which would most likely be as a cook and would likely own his own small restaurant. I you have sirzechs and grayfia go to eat his food and have them become friends with each other.
In the supernatural world have him act as a mercenary or enforcer for hire, that would give him some experience in the dxd world and against the various supernatural beings in it. this would have alot of gaps that the author would have to fill because I can't think of them.
this is lost avalon signing off.
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ideas for stories
Randomthis is a collection of ideas for stories that I have gathered for a long time. I have put them here for others to use for their own stories. All that I ask in return is that you let me know what ideas you are going to use.