Arthur is the once and future king.
They can finish up this story.
Then Merlin and Arthur can save the 21st century from some dread new evil or just completely plagiarise the Camelot 3000 comic books....
Arthur comes back in 1897 to fend off HG Wells war of the Worlds.
Each season could be set in a different era.
Arthur returns from a big sleep in Avalon and Merlin is immortal.