Think of This Challenge as a Game where you take decision and these decisions lead your character into certain events that will change his life.
This Issei be the same as from the start from the novel he will do the same things have the same background meet the same people everything normal. But the only thing you will change is what if he didn't take a certain decision that changes his life in canon.
But all of you will think "if he didn't take that decision his life would never change" the truth is that every decision we made define us as a person he not taking this decision will lead him to a different path with a different end also that will change his perception and personality to a certain level all depending on the events that will happen next after he takes his decision.
AN:
This Idea came to me after remembering Fate Stay Night routes the character start the same way with no change of his background but in every route he makes a different decision that will lead your character to a different end.
And again this movie M.R. Nobody if you haven't watch it recommend its awesome how just a tiny decision change your life.
And Yes I don't like OP characters unless they have suffered through their life making sacrifices for just there objectives. Because no pain means no gain
Also All of you are wondering why I don't write a Fic myself the answer is simple I have my grammatical isn't good also that I can't make detailed atmosphere for the story saying every expression the characters do and that I lack of motivation to write it.
Recommendation: Chrono Crusade (Both Manga and Anime have different ending and both taste differently)
If you are interested in this idea please PM also Please leave a review to see what so that I could read what people think.