Random idea Fate x Hayate the Combat Butler
After the end of UBW Shirou go to Clock Tower with Rin, but as he senses no progress he leave the Clock Tower and begin to work as a mercenary.
However, one day he received a request to act as a Bodyguard and a Butler for Saginomiya Isumi. Obviously, Shirou refused at first as that will limit his activity. However, the Saginomiya manage to convince him via blackmail or something. Like maybe Kiritsugu owes them a favor or something.
From here Shirou will be involved with the adventure of Hayate and learn about Royal power.
The timeline is up to you.
You guys can pick before Hayate becomes Nagi Butler or after.