Since I don't know the exact 'first fall of equestria story', I can't say for sure. But it could be the main pony characters like Twilight Sparkle, if it's related to the My Little Pony universe. Maybe she has to face some challenges that lead to the fall of Equestria.
Well, without having read the 'first fall of equestria story' in detail, I can only guess. It could be about the fall of the equestrian society. It might start with internal conflicts, such as political disputes among different pony tribes. These disputes could lead to a breakdown in the magical balance that holds Equestria together. As a result, chaos ensues, and the land starts to crumble, with ponies losing their homes and their way of life.
The main characters are likely to be the Equestria Girls themselves. Twilight Sparkle from the human world would be a central figure as she often takes the lead in many situations. Then there are her friends like Rainbow Dash, Pinkie Pie, etc. Also, the pony versions of these characters might play important roles as they interact with their human counterparts.
Satire is a big characteristic. They often use humor to poke fun at the systems or groups that are in decline. For example, in Evelyn Waugh's 'Decline and Fall', the upper - class British society is satirized through the comical misadventures of the characters.
One interesting plot point could be the culture shock the Equestria Girls experience when they first arrive in Equestria. They might be amazed by the magic that is so commonplace there but strange to them. Another could be their interactions with the ponies. For example, they might have to learn new ways of communication and social norms.