So, this will my first idea back from the temporary hiatus I was on before. The hiatus only ended up lasting around 2 weeks, because while it's true that I'm having difficulties coming up with new ideas, I have decided I will instead just update less frequently to solve that issue. I'm looking forward to explaining this idea, by the way.
I basically want either Shirou or Archer to exist in the world of One Punch Man, and for either one of them of replace Saitama. I want Shirou/Archer to be as powerful as Saitama is, but with swords, and you know, a lack of the boredom that Saitama experiences. Shirou/Archer also have far more proactive personalities than him, so there's also that. I want one sword to be enough for most enemies, and maybe a lower ranked noble phantasm for more difficult enemies. Perhaps have the higher ranking noble phantasms still appear for intimidation uses, though.
To be honest, It'd make the most sense if Archer was the character replacing Saitama rather than Shirou, just because it makes more sense given he has far more experience and is a counter guardian. That isn't my bias for Archer speaking, it's just that he has much more capabilities than Shirou, even post UBW. Plus, it'd be interesting to see the contrast between him and the other heroes, since Archer acts the way he does.
That does not mean that I wouldn't be interested in a Shirou-centered story. My bias for Archer has settled down quite a lot since I last wrote of it, and I'm much more willing to do ideas relating to Shirou nowadays. It'd be just as interesting to see a story focused around him as it would be with Archer, in my opinion. He would act more similarly to Saitama than Archer, minus the boredom, however Archer would also prefer to stay in the shadows, whereas Shriou might actually join up with some other heroes or even the hero association, depending on the circumstances.
(From now on I will be referring to both Shirou and Archer as Shirou to avoid having to write both of their names every time.)
I don't think there's really a reason to write much about the reality marble (UBW), given that it will pretty much never be used. If one sword is enough for most enemies, a low rank noble phantasm is enough for the more difficult enemies, do you seriously think Shirou would ever have to actually use UBW of all things? At most he might get to one line of the aria, and even that's a huge stretch, given that even that shouldn't really be necessary at any point in the story.
You don't have to have Genos become Shirou's disciple, but if you want to follow One Punch Man canon even loosely, it may be necessary to do so in order to ground things to that canon. I think that even so, it could be probably be worked around, but that's up to you.
The main driving point of this story would be Shirou's personality and how it affects the characters and plot around him. If you don't focus on that, this just becomes a carbon copy of One Punch Man's original story, but with swords instead of punches.