Board Thread:Spoilers!/@comment-32390199-20150823222638/@comment-24674051-20150902141640

I honestly don't think that you could properly pull off an Arthurian storyline without a few basic elements

 Love triangle between Arthur, Guin, and Lance Excalibur and the Lady in the Lake The Round Table The Quest for the Holy Grail (Or at least the knights involved in said quest) Morgan Le Fay and Mordred 

They have already addressed the Love Triangle, the Round Table, and at least the main characters for the Holy Grail (and I suspect there will be some variation of the Grail itself). Excalibur has already been hinted at in Season 3, so I feel confident they will address that as well.

That just leaves Morgan and Mordred. There is just way too much potential there to NOT have some part of the story include these individuals. My guess is that they will show up, and that may be where the writers introduce one of the twists to the story.