Well, since we're on the topic of fanfics, a thing I never seem to see is a sort of 'Earth human comes to equestria' taken to a certain conclusion.
The conclusion being that reality itself
should not be doing that, forcing the human to shift into sort of a horrific human pony hybrid and endangering the wall separating the two dimensions(This would cause the planet Earth to crash into the Equestrian Plane[Equestria is flat{Discounting natural terrain} and extends in all directions infinitely]). The human desperately tries to hide his affliction along with the mind shattering pain caused by reality periodically trying to 'fix' itself and causing human to undergo painful pony/human randomizations. All the while, Celestia has the main (Yes 'main', the goddamn replacements of words irritates me to no end) 6 watch over him because she feels something strange about him, causing massive
morality dissonance arguments between them on issues such as omnivorism, magic, and, say, religion ("Your gods actually exist, ours don't."). Until the end he should retain his disguise as a regular pony (Full body cloak thing?)
Maybe at the same time, a Pony crosses to Earth and goes through the same thing.
At the end he goes batshit insane due to the stress or something (Idea, he gets infected with both NMM's Corruption and Discords Reversal but they take a while to fully effect him due to mismatching mental software[Say organically adapting a Windows Virus to a Mac] and manifest as voices in his dreams), reality tries one last time to stabilize and he becomes an alicorn centaur who then tries to Break down the wall. Then there's an ending.
Wow this turned into a brainstorm thing.