Bridle Gossip=Twelfth Night gets my vote. Everything is inverted for a while and poison joke in general allows for all kinds of crazy role-swapping. Plus I know you all want to write it.
Someone should do Celestia as King Lear as well, even though it doesn't match up with any current episode at all. Could also try one of the Mane 6 as King Lear and then use the CMC as the three daughters for the lols.
Over a Barrel=Romeo and Juliet. Bloomberg gets to be Romeo, Fluttertree as Juliet