Fallout: Equestria is a fairly large and fairly popular MLPFiM fanfic. It uses the general narrative structure and setting elements to create an action-adventure tale in an alternate-universe future Equestria where a new generation of ponies are dealing with the repercussions of the actions of the world leaders (main six and princesses) since passed (with some flashbacks).
Actually, I was pointed to your tumblr by someone on the Canternet #FalloutEquestria IRC channel who had seen the balloon thing and was like "Retl: Look! Popping!" and I was like. : O and then I clicked it and hit the like and fave buttons on Tumblr/Derpibooru/dA.