well here's an idea: subaru in another world! i've recommended characters come to re:zero, but not the other way around. with this one, i challenge you to break the norms of crossover fanfiction.
re:zero, was, at least in the beginning, heavily laced with satire on the fantasy world genre. so if you do one of the first 3, or this one, don't follow the tropes set up by normal fanfiction. don't do it. do something new. let's say subaru goes into another world, don't have him explain his backstory! seriously! don't! we all know the damn story! try to make the characters stay real as they would be in their own shows. how about when subaru finally reveals he's from another world, you get a variety of reactions.
i've always found this to be true: any good cast of characters at the very least should be functional in a sitcom. you know why? because you need differences in character, basic contrasts, that's what people are like. let's say subaru winds up in...naruto, why not. so like 5 chapters in (please) he tells a couple people he's from another world...from another world. (like my idea is that after killing betelgeuse he gets pulled into another world again, and you can have a bunch of humor with that where he's not at all phased by anything for a few chapters. just like, oh boy, a new world...great.) so he reveals he's from a different to...shikamaru, naruto, sakura, and choji. Who believes him? what are there reactions? shikamaru would accept it given enough proof, if ino was there, she could look into his mind, i think choji and sakura would both be skeptical but more willing to believe him. naruto...well depending on what point in the series this is in, his reaction might seriously change, actually as well as sakura's.
but do you get my point? put subaru into a different world, but do something unique. if you're putting him in naruto, how about he gets a secret from someone, so after he dies and he can tell them, they know he's telling the truth. how about he has to convince shikamaru to teach him shadow justus, or maybe he doesn't have chakra at all? what if he comes just the shinobi war breaks out and he has to convince them to help? how about he's younger, and arrives in the world in the academy, with his jutsu being this one. how about something original and unique!
I challenge you to create a unique subaru goes to naruto world (or some other universe) fanfiction!
p.s. if you do write this, and if he does go to another world, especially naruto, tell me who his checkpoint person is, like the apple selling guy, who do you choose? or just tell me who you would choose otherwise.