The main issue with this first idea is that the idea in question is too similar to a pre-existing one, so I'll try to allude to several differences from the original that you could use in your story. The fan fiction this idea is based off of, but hopefully different enough from, is a really good crossover between Fate and Highschool DxD known as "The Holy Man of the Church Creek." I'm not writing this because I think that fan fiction wasn't good enough, it's just that I want more content relating to what they showed in it. The second idea is an original idea, but both are rather short, hence why I combined them into one chapter.—————————————————————————
I want Emiya Shirou to somehow become a mix of the God from the Angel faction, a human with UBW and tracing, and having the most powerful demonic powers that could realistically exist in the world of Highschool DxD. How this happens is up to you, as is what he does with those powers. You can also cut out the demonic powers if you think the story would go better without them, or if you just don't want the story taking the route that would inevitably lead to. (Both of those biblical factions being lead by Shirou.) If you really want to make the story unique, perhaps make it Archer rather than Shirou with those powers.
Because of the fact that Shirou would have become God, he should be able to trace divine constructs, making him all the more powerful. I do not want him joining any peerages, but I do want him to do several things for the biblical factions, such as adding to God's sacred gear system, or curb stomping Kokabiel.
—————————————————————————
For the second idea, I want UBW to become a sacred gear in a sense similar to Archer's arm in Heaven's Feel, but without the whole "Killing the user over time" thing. In other words, the holy shroud would still work as a seal over the abilities of the arm, and the arm would still be the access point for UBW, but it wouldn't be invading the user with the reality marble or making swords grow out of them.
It's up to you whether you want the reality marble to be deployable or not, and it's also up to you who to give the arm to, though I'd recommend either a less perverted Issei, Kiba, or an OC. The arm should make the user more Shirou-like or Archer-like in terms of personality, but the reality marble should be unique from all forms of UBW we've seen in canon, and suited to the user instead. (If you have it be deployable.) Optionally, you could also have Archer as a guide inside the sacred gear, and for the dream cycle to occur with Archer or Shirou's memories.