Something horribly happened to Equestria, Discord sucked all the power and energy of the Elements of Harmony and used them causing to fire an enormous laser causing every land of Equestria to break and split apart with a powerful beast from the center of the land. Rainbow Dash tries to foil Discords plans, but fails and the ray, that Discord used, drains the Elements of Harmony and has the unforeseen side-effect turning Rainbow Dash into a beast called a warepony, that gives her greater strength and abilities only at night.
Now that every land of Equestria has been broke apart, an evil beast that will be awaken in a few days, and Rainbow Dash's powerful warepony form, Rainbow Dash needs to find a way to put this jigsaw puzzle land back together with a help of Twilight, Pinkie Pie, and a friendly creature, who lost his memory and Rainbow Dash will help him get his memory back, her awesome adventure is about to begin.
So... it's a decent idea to take elements from Sonic Unleashed and apply them to Equestria... the floating continents, the different cultures, you could do a lot with it.
However, it appears that what you've done is simply taken the entire opening cinematic and just replaced characters from Sonic with characters from MLP...
While the idea has a lot of promise and could be something special, just retelling the entire game script with switched out characters really isn't a good idea, it becomes just a cheap knockoff.
Try using the plot elements from the game and fill in the gaps yourself... after all, you have a world torn apart by an egotistical villain, a spirit without a memory in need of a friend, a dark spirit of destruction and a world spanning adventure... there's a lot of room to tell a good story especially splashed with mlp.