Ash-Greninja

Greninja could turn into the Ash-greninja if it spent a long time within your party and then it could evolve into Ash-greninja or maybe it transforms into Ash-Greninja in battle like Xerneas or maybe it could be a rare trait to froakie.

But... you aren't Ash.

This has already been rejected. We're not Ash, so it makes no sense why we should be able to obtain Ash's Greninja.

oh...did not think about that

You could spray a bottle of Ash's branded cologne all over your poor Greninja, and as he coughs and dies from the fumes, he painfully metamorphosises into Ash-Greninja, since he is now touched with Ash's essence. In all seriousness. Ash-Greninja is not happening. :p

I understand we're not ash so we can't get it, however we're able to get it in the core games, so it could be possible we earn it through an event, where we earn the Greninja from "Ash"

Yeah, still no. Doesn't matter how it's obtained; the concept of Ash's Greninja's is rather dumb - even if canon. It doesn't have a place on Pokefarm.