In some ways, this could work similarly to the Fate/Grand Order crossover idea I presented before, but in other ways this could work in a totally different manner. Instead of involving Ichigo in the Fifth Holy Grail War, however, I had a different, more unique idea that also borrows elements of Tsukihime and its story. Basically, this would be the premise: Ichigo, post TYBW, encounters Shirou Emiya, post true ending of UBW, while he's in the middle of facing a dead apostle. Ichigo helps him out, and Ichigo and Shirou then exchange names before asking each other about their respective powers and abilities. For scaling purposes, Ichigo would be far stronger than Shirou and most dead apostles, however Shirou would be able to make up for this with his tracing abilities, though they obviously wouldn't work on either of the Zangetsu blades since they have souls in them. Going back to the premise, Ichigo and Shirou end up confusing each other, because they have no idea what the other is talking about, and eventually Shirou has to go back to the Clocktower to meet up with Rin. the story of Tsukihime would then begin occurring, and one day Ichigo and Shirou happen to be in the same area, and meet up again. I don't know much about Tsukihime, but wherever it is that the story takes places in is where the two happen to meet up, and they end up staying awhile in that location. You can go wherever you'd like after that, maybe even find a way to incorporate servants and characters from the Fourth and Fifth Holy Grail Wars into the story somehow, such as Iskander and EMIYA, or Waver Velvet and Illya. I'd also like to see some events take place in the Clocktower, maybe as a kind of side story that takes place via flashbacks of Shirou.