Disclaimer: I don't own Merlin
My take on what would happen after the series ended.. So yeah.. Review and such ;)
Percival had followed the footsteps Arthur and I had left behind on our way to Lake Avalon, and then where the foot steps disappeared, he just hoped he was going the right way. A few hours after I had pushed the boat through the lake, Percival had appeared, knowing, just by the look on my face what had happened. On our journey back to Camelot, I told him the entire story. I told him all of the times I had saved the stupid prat, and the prophecy of the Once and Future King. We made it back to Camelot, and Gwen became Queen. Time passed, and she lifted the ban on magic and made myself the Court Sorcerer.
I worked as the Court Sorcerer under Gwen's rule. She, as much as it pained her to do so, had to remarry for there to be an heir. She married Leon, for he was Arthur's most trusted knight and they had two beautiful sons. Gaius passed away only a few years after Arthur. After I had become the Court Sorcerer, Gaius had taken on a new apprentice, whom then took over as Court Physician. Gwen's sons grew, and all the knights and Gwen herself grew older. I, on the other hand, didn't age at all. Percival died, as did Leon, and Gwen, and everyone I had known when Arthur was alive. I had still not aged a day. I accepted a long time ago that it seemed the Great Dragon thought I was immortal, or at least destined to live until Arthur returned. As such, after Gwen's death, her eldest son took her place as king. I kept my position for awhile, though I decided I should travel and better learn my magic. I appointed a new Court Sorcerer, and began to travel.
I didn't stay in any places for very long, mostly because I didn't age. I would come in to a village and become an apprentice and learn their craft. I did this with Druid villages as well. In the Druid villages, I learned even more magic and became more skilled. As time passed, magic became less and less common. Many of the magical creatures naturally went extinct, and less people practiced magic. I lived through many wars, and I studied at many different universities. I learned many languages and acquired quite a wealth. No matter where I traveled, I tried to keep in the background. I saw Camelot and Albion grow very strong, and then crumble. I knew history better than most, and could speak many languages fluently. No matter where I traveled, I was able to fade into the background.
I am currently in London now, I own a house on the outskirts of the city. The countryside, is very beautiful. I own a few horses, which I ride often during my free time. Though my main reason for moving to London, is the same reason why I feel that I am still alive today, Arthur. I could feel that he had returned, and my magic had brought me here. Arthur, having been completely reborn, now went to a high school in London. He is about the same age he was when we first met, and as such, I'm taking that as a sign to restart our destiny.