(I'm tempted to claim that Queen Elizabeth the Tenth of the starship UK - in the 'Doctor Who' episode "The Beast Below" - was descended from Guinevere because they were both black. But that would mean moving one of them to the other's TV dimension, and I refuse to cut 'Doctor Who' loose from Earth Prime-Time. Somebody in the family tree of the Royal Family's future lineage will just have to marry someone of a different skin color.)
But at least when the series ended, 'Merlin' hued fairly closely to the established legend. Morgan Le Fay and Mordred were both killed in battle, and Arthur was mortally wounded. Merlin (who by rights should have been removed from the scene by this point, entombed in an oak tree) threw Excalibur back to the Lady In The Lake to keep in trust. And Arthur was sent in a bier-boat to the island of Avalon - with the promise that one day, in England's darkest hour, he would rise again.
Even though other details would be different, I believe this promise holds true in Earth Prime-Time as well.
But I'm not going to waste print in arguing that any other Bradley James' character could be Arthur Pendragon Reborn, because they would be from different TV dimensions. When King Arthur does return, I think he will be reincarnated with a new body and a new name. We might never even know that he was the Once and Future King when we see him again.
For alls I know, he might already have returned in the Toobworld timeline and we didn't even realize it. Wouldn't you consider Great Britain in WWII with the London Blitz to be England's Darkest Hour? Was there a TV hero for that time period who could be considered such a stellar champion of the realm that he might as well have been Arthur Reborn?
This would call for more research into the TV series which were set in England during WWII, especially those from the 1950s and made in the UK.
Many classic literary characters have been updated to modern times for television: Sherlock Holmes and Dr. Watson, Fu Manchu and Nayland Smith, the Invisible Man, and Dr. Jekyll and Mister Hyde. (Tarzan has been updated several times, but as with Jekyll, I'm hoping the case could be made that they're descendants of the Edwardian original - should that version ever get a Toobworld adaptation.)
So it's pozz'ble, just pozz'ble, that one day the legend of Camelot will be adapted for the present day. This would mean that all of the characters from the legend would be reincarnated, not only Arthur, Merlin, and Guinevere but also Lancelot, Gawain, Galahad, Parsifal, and the servant Patsy. (Okay, maybe not.....)
The comic book series "Camelot 3000" could serve as a template for how this could be achieved.
But for now, we'll just assume that the return of Arthur Pendragon is yet to come, and that somewhere out there a white-bearded wanderer maintains his vigil, awaiting the return of the Once and Future King... at least in that TV dimension.
Which reminds me - the presence of Merlin in present day Earth Prime-Time need to be addressed at some point.....