Twilight Sparkle gets a mirror portal from Celestia, but it lacks a link to a specific world. When Twilight begins to experiment the portal using her new increased magic reserves, Rainbow Dash tells Twilight of her hurting ego. After begging profusely, Rainbow convinces Twilight to search for a racing competitor across the dimensional border. The meeting that occurs will spark a (mostly) friendly competition that will test the friendship between three worlds.

And perhaps even decide their fates.

My first crossover that happens to involve three of my favorite media franchises and as such, it needs a disclaimer.
Disclaimer: This work is purely for non-profit purposes and contains characters from copyrighted works. Team Fortress 2 is owned by the Valve Corporation. Sonic the Hedgehog is owned by SEGA. My Little Pony is owned by Hasbro.