Not necessarily a showdown. Not unless things turn out that way. I'm likely going to put some sort of greater threat in place, likely one of those true demons or demon lords I mentioned, but at the start my character is going to be an anomaly, something that shouldn't exist the way he does. I don't want to elaborate to much for fear of spoiling, but at the start, either the church, the clock tower, or both will want to be able to keep an eye on him at the very least. That is, once it's clear that he's such an anomaly.
I'm also willing to bring in canon characters, should anyone want to role play them. Just as long as they can be explained, as this will take place after tsukihime and fate/stay night