Oh man... I have had some crazy Wild-Ass Guesses in my time. This one takes the cake.
Original Merlin Timeframe:
Since the beginning the outsiders have been a huge threat. Starborns have risen up and fought them, but some also die tragically early. Not all of them have the power level, skill, circumstance, training, or anything to really prepare them. For one or more reasons they fail. They are not enough.
Merlin comes along, full of power. He's strong, studied, and the epitome of wizard. Mostly. Hes running with at least one knight of the cross (Arthur). He is not a starborn... cruel twist of fate. He wasn't born at the right time, or some others were born under the circumstances and not he. If he had been, with what he was, the war would have come to an end. It was the ultimate tragedy. He does a ton of impressive things, dabbles in all the schools of magic, and eventually even creates demonreach to imprison outsiders, and other threats. He becomes the first warden, and at some point even binds Arthur (The brit) into demonreach. He (arthur) is the once and future king. Arthur needs to be around when the outsiders are defeated, or there is a cure for nemesis, or any other number of reasons. He puts him on ice. Breaking the laws of magic to make demonreach, and possibly even other things (Before there were laws!) he becomes corrupted. If only he had been a starborn. Demonreach was created however, to imprison the outsiders. All of them. Especially their leaders the Old Ones. To wield it properly, one needs to have a will strong enough to actually overcome the being you wish to trap. Gods? Merlin might overcome one with his impressive will. Outsiders? Not any significant ones. Only a starborn can properly touch the mind of the outsiders without being driven insane. Merlin wasn't a starborn, and it was his single unforgivable flaw. He couldn't be the proper warden.
But starborn are born semi-randomly. Every 666 years random persons of varying levels of power are born as starborn. Not all of them are capable of doing what Merlin needed them to do. They needed to be strong of will, and strong of magic like Merlin himself. Only Merlin, he himself felt, would stand a chance.
So he devised a plan. He utilized reincarnation magic. He would thrust himself into the future, generation by generation, until either the threat was dealt with or else he was born as a starborn. In each life, he would be destined to find his way to demonreach, take it as his sanctum, handle what cosmic business need be handled, and learn the knowledge of his past lives. If he was the starborn he would be able to properly end the outsider menace.
--Fastforward--
Through the years both the starborns and the merlins were born, but they never quite managed to sync up. Merlin really wasnt living life backwards, he was living life, over and over again, into the future. He was many famous wizards through the years, but never the starborn.
The gatekeeper is probably a title often reserved for the eldest starborn on the council who takes the job. Its purpose is to use the power of starborn against outsiders at the outer gates and its why rashid isn't ever around much. He doesn't have the knowledge of merlin or the power of merlin, but he using what he does have to hold the gates.
There were blunders. In one lifetime, Merlin was born as Heinrich Kemmler. Not a starborn, but a reincarnation of Merlin, with all of his power. Kemmler was corrupted before he ever made it to the island, and when he became its warden he used magic to make a copy of his brain (Bob) a construct of air that would hold all of the knowledge of Merlin. He separated some of his knowledge into Bob that he did not want inside himself, and to prevent the Merlin personality from taking hold. He planned to use necromancy to stay alive forever. This was a critical issue because he NEEDED to die in order for the universe to re-roll the slot machine and pass Merlin's power on to a future generation. The 666 years was coming close. It took a long time for the white council to kill him for good and free the reincarnation, and in the process Bob the skull was snuck away by Justin DuMorne, who himself was corrupted. The outsiders gained possession of much of Merlin's knowledge and Bob even hid and suppressed some of it. Justin figured out who the next starborn was to be and planned on stealing him away for the outsiders to turn into a destroyer. He used the knowledge in the skull to identify two new, vulnerable starborn (Harry and Elaine). One of them might even have Merlin's power. To see, he would need to test their magic against an outsider (He Who Walks Behind). A normal untrained starborn wouldn't have enough power to deal with a walker, but a Merlin might.
Eventually Merlin devised the pattern and found exactly what would cause a child to be born as a starborn at the proper time. He engineered his own rebirth, died, and created Harry Dresden, who is both the reincarnation of Merlin and a Starborn. Harry has been too busy to really explore everything the island has to offer, and hasn't yet gained his past memories.
Some on the white council know about his breeding. Mab pointedly mentioned to Harry that should he ever find himself immortal he would do well to remember that immortality is no substitute for intelligence. The white council fears Harry because hes basically Loki. Causing all kinds of trouble. He has the power of merlin (He was born with tons of power) but none of the refined skill yet, Hes very young and untrained. With his temperament, and breaking the laws, they fear he might be the next Kemmler and they might not be done rolling the slot machine. They were constantly ready to execute him in the hope that a new starborn could be created with Merlin's reincarnation, but it was foolish. The wiser members of the council decided to raise him up as a powerful wizard while monitoring him. Uriel and Mab both vested even more power into him.
Hes not ready, but the outsiders know they have to push hard and fast now, and try to remove Harry indirectly by treachery or by corrupting the council and it's purpose. It's their only chance. So Harry has to get ready. He has power, even if he lacks the skill of age, and he has a strong will. To win hes going to need to do what no wizard, merlin-reincarnation, or starborn has ever done. He has to cage all of the old ones. If he dies, it may be a very long time before someone is born with both Merlin's Power and the Starborn's potential again, even if the council knows the "formula". Best case, 666 years. Do they have that long? Probably not.
This also means that while every white council "Merlin" isn't necessarily a reincarnation, many of them could have been. Arthur Langtry is not of course. The title honored merlin reminds them of their purpose.
Gatekeeper and Merlin then, could be filled by any wizard with enough knowledge, but they serve a ceremonial purpose as a place where the failed experiments (merlins and starborns) ended up. If kemmler hadn't been such a nutcase and traitor he may have been given the position of Merlin until he needed to die. Instead we got Arthur Langtry.
