Here is a thought for a oneshot I am considering writing, but alas my type-moon knowledge is sorely lacking.
Rin's actions fail to make Sakura leave from the Shadow and she consumes even more resulting in her becoming a threat to humanity. The Counter Guardians (our protagonists) are sent to deal with her. Its really vague at the moment, so it obviously needs fleshing out. It would be a horror Problem is I am trying to decide which specific Counter Guardian (other then Archer)
Oh yeah, before I forget... Yes Sakura is the villain and is Chaotic Evil, and starts an apocalyptic countdown.