I love these questions!
Okay, what about: Trixie returns and kills Rainbow Dash. Everbody in the mane six then realizes that Trixie has turned into a demon and they have to hide in the libary basement. It's like a horror movie, everbody gets slaughterd except Fluttershy because she is best pony.
I need more ideas for this episode, PM me if interested! (No seriosly, don't)