One a young red headed 3rd rate magus with dreams of being a hero of justice the other a green haired weak teen in a world of superpowers determined to help others both willing to risk their health & lives to achieve their dessires. Is it any wonder Emiya Shirou & Midoria Izuku turned out to be the same person?
requirements
1. Izuku is the reincarnation of Shirou & as time goes on it shows (appearance changing slowly, having his magecraft, etc.) But still being different (using his brains, his circuits not being atrophied from disuse, &/or having a greater sense of survival than Shirou).
2. Friends & enemies from Izuku's past life start appearing either as they where &/or reincarnated similar to him if so then they have quirks that compliments their past lives abilities. They also can't be canon characters (like Rin being Momo) unless they are an obscure character or they are so similar it would make sense they are the same person.
3. Harem the bigger the better (with most versions of Shirou's seemingly a magnet for pretty woman & canon Izuku probably would have had several girls if he was more confident & his world didn't place a person's value on quirks) made up of woman from his past & current lives.
4. Izuku's magus abilities having some differences in this life (because his circuits never got a chance to atrophy, his personality & sense of self is different, & such)
optional
1. Izuku having a quirk (if the other reincarnation have them I figure it would be fair if he did to but that's up to you).
2. Izuku gaining one-for-all or not. If so how would it & the remnants of the past holders interact with his magecraft.
3. What version of Shirou is Izuku the reincarnation of (be it one of the routes, spin off series, your own version, or a mix of them).
4. Lemons.