This is up for grabs. I wont be making it but here is an idea i want to Share.

Back before Equestria the ponies were few and managed to move along thanks to their friends Megan, Danny and Molly. One day however Danny had a disagreament with his sisters and went home. Because of an event Megan and Molly were stuck on the pony side and were never heard from again in their world.

Time pased and in order to protect the ponies Megan and Molly were reborn as Celestia and Luna and helped the pony race prosper but still remembered their lives as humans and miss their brother, also main ponies like Twilight, Surprice, Wind Whistler, BowTie and Firefly became stone, to protect them and only when the 3 siblings were reunited the curse on those ponies would be broken.

As for Danny life was almost hell. One day robers entered his house and killed his parents and burned the farm. Danny the lived homless and in the streets, there he meet batman while he was robing the bat movile(yes he replaces Jason Todd), he trained under Bruce and became the secon robin but fate was cruel to him as he was kidnaped and killed by the joker but later revive leading to him Becoming the Arkham Knight and have a vendeta against his former mentor.

The evets of the Arkham Knight took place and Danniel became Red hood(Arkham knight design), during his criminal huntings he was transported to equestria. The everfree fores to be exact, were he defeated some monsters from there, but at the same time breaking the curse on the old ponies and revealing to Celestia and Luna that their brother is once again in equestria.

Also time moved faster in equestria, because for Celestia/Megan and Luna/Molly it had bean more that 1000 as for danny it was around 20 years