SplendidPuma
maybe do something like this an epic battle at the end of the quest and he gets a reward and that could be a grail that grand a single wish. when he then says his wish is "bring me to the universe, where I belong to" that was his mistake, the keywords 'belong to' as in archer, where archer belongs to is the nasuverse so you can send him as a heroic spirit since he did the quest that should have saved the game world and all its citizens and that qualify for him to be sent to the throne of heroes or you could send him to Britania in the past where he can meet Artoria or in the modern world or a driffen time where he wonders the world and then stumbles on the holy grail war. what else to say, I didn't read fate only saw one anime of it with the holy grail where Gilgamesh is the first summoned so I don't know more than that. and for romance and stuff you could do is, maybe he can be her knight in shining armor, anyway it's just an idea