Disclaimer:Probably a completely unoriginal idea but whatever.

After the Seven Singularities and the defeat of Solomon and Goetia, Ritsuka Fujimaru lives out a normal existence as best he can under the watchful eye of the Mage's Association.

One day however, he wakes up to find himself once again trapped within the Moon Cell, except this time it's not just a small section of it that he's confined to, and even more distressingly, it seems that the one known as Kishinami Hakuno does not exist in this timeline, and he is the one stuck taking his place.

Hopefully BB and the other Alter-Ego's don't take his presence(and Hakuno's lack of presence) too badly...

Oh who is he kidding, they're going to be stark raving mad when they find out about what happened.

Nitty gritty details time.

Takes place around half a year after the defeat of Goetia and Solomon, (Lost Belts/Epic of Remnant never happened) and right at the start of Extra.

In the Extra Timeline that Ritsuka ends up in Hakuno doesn't exist.

The events of CCC will take place, but things will happen differently than in canon because of Ritsuka's actions in Extra.

I'm thinking of either having Ritsuka summoning either Archer Nobunaga or Saber Okita, might have him summon Caster Gil or Scathach=Skadi instead.

Ritsuka will be fighting the Master/Servant combos that Hakuno did.

Ritsuka won't be able to Code Cast, but his Command Seals will replenish themselves after a certain period of time has passed. Once CCC events happen he'll be able to Code Cast though, at the cost of his renewable Command Seals(He'll get a warning once that happens though, so that he doesn't waste all his command seals during a particularly grueling battle).

Alaya won't be trying to erase him from the Timeline or send Counter Guardians after him, as it realizes that without Ritsuka or Hakuno's intervention the timelines are screwed.

Rin and Ronnie play the role that they originally had, but their interactions with Ritsuka will be much different than their interactions with Hakuno, as will the other Masters.

And that's about it. Probably about as cliche as a fanfic idea can be, but oh well, not like most of my other ideas are original either.