It has occurred to me..

...after watching and listening to Battlefield for the bazillionth time that, that... Eleven should be Merlin. No, seriously. He is like Seven's younger, slightly scarier brother. Also, he's currently going through a thing where he doesn't want to shout his name from the rooftops anymore. And 'Merlin' is as good a name as any.

The only difficulty would be finding an actress who could pull off a young Jean Marsh (then again, if we could just de-age Marsh, we'd be set)

(Then, of course, River can show up to rescue him from the cave wherein he gets trapped at the end)

Of course, my personal crack theory is that Ace shows up and is all "Haven't you dealt with this yet?" and disapproving at him, so they team up to go rescue Arthur and start the round table and stuff.