Spike is searching for the perfect ring, with the intention of finally popping the question to his long time girlfriend. Thank goodness Trixie knows of a place that carries totally not cursed rings worthy of a pony like Rarity.
Ember is very proud to be the dragon lord, and very satisfied with her life, however, by accident the "innocent" Crackle will take everything from hir, maybe forever or maybe not.
A Cozy Glow-hating brony is fooled by the nefarious filly herself into switching bodies, now he's the one locked in Tartarus while she's causing havoc in his world. Uh-oh. Also, nopony believes him... because he's Cozy Glow. Makes perfect sense.
Spike woke up to find himself as his pony caretaker, Twilight Sparkle, while the original Spike somehow still acted like Spike himself. The now-pony decided to go about fixing it alone, with both expected and unexpected results.
A dragon assistant toiling away in the Castle of Friendship suddenly found himself as a familiar unicorn fashionista. What's more, he was shocked to find himself stranded in a critical moment of the not-so-distant past.
A human of low character dies and gets sucked into Equestria as a copy of Twilight. The new mare then proceeds to cause chaos for the ponies, but she tries to help... and, more importantly, make some money
An unwilling non-brony traveller from Earth finds himself switching between bodies of the Ponyville locals randomly, leaving so much chaos and confusion in his wake that Discord might as well retire now.
Sunset Shimmer returns to reclaim her place as Celestia's student, how you ask? by Stealing Twilight's body! only... things don't exactly go as planned.