Following along from thoughts on the end of Merlin:
What would qualify as a “time of greatest need” and deserve King Arthur returning?
The Second World War. Particularly the Battle of Britain... but how do you get him up there and involved?
The English Civil War... but who would he fight with?
I’d be tempted to go a slightly different way, and put him in an urban fantasy kind of position, fighting secret apocalypses unnoticed.
I’ve also previously had ideas about Arthurian survivors in the present (and who would they have to talk to but each other?) and even wrote a novel about the Sword in the Stone being rediscovered in the present.