Once Arthur comes back, Merlin will film EVERYTHING.
Merlin knows he will survive Arthur, no matter what. But sometimes, he's so busy trying to keep the memory, that he forgets to live them.
Arthur is annoyed at first because Merlin keeps holding that rectangular lightning brick in his face and asks him to stop. It takes a long while for him to listen.
But once he realises what Merlin is doing, Arthur keeps a video log. He uses it every night when Merlin is asleep, and sums up their day, confessing his love long before they even get together. Sometimes he hides a camera in their room, so they can live life normally, but Merlin will still keep the memory.
Arthur lives a long healthy life and dies a sudden death at the age of 98 in Merlin's crying arms. His will leave Merlin terrabites of memories.
It's so many that Merlin can't go through all of them before Arthur reincarnates once more :)
