Let's shake things up a bit. We know how things go in our timeline; what happens and when and to whom.
SO. What about a MIRROR universe? Would Arthur be the servant and Merlin the Prince? Would Morgana have grown up with Arthur or would she be the Morgause of the tale? Would Morgause be the Morgana? Would Uther be the King and Gaius the physician? Would gaius have retained his magic and used it to aid the King whilst also purging Camelot of it's magic users? (Oh, wait.)
Whatever you'd have happen, let's visit those universes!
(Note: This isn't an AU challenge. This is an AT challenge: Alternate Timeline.)
Go with it!
We'll snap back to reality at 22:00 GMT on the 2nd March 2010