There are at least half a dozen fics on fanfiction.net alone where an OC soul gets implanted or incarnated in Matou Shinji's body, and... why? Why does this very specific plot device keep getting applied to this specific character? It seems like a sort of indecisiveness between wanting your protagonist to be a canon character or an OC. If you don't like how a canon character turned out (and in the case of Shinji, I'm 100-percent with you on that) and would like them to be the same person but different, there is such a thing as character development. If you want an OC, why not just make a straightforward one rather than making up new plot mechanics to introduce a character who's an OC in all but name?