Merlin (TV series)/WMG

Merlin and Harry Potter take place in the same continuity
It is established in Harry Potter canon that Merlin exists and that he was one of the most powerful wizards imaginable. Also in Harry Potter, a lot of the magic is ebbing in quality, which would make sense given the absence of wands in Merlin. Some wizards in Harry Potter can perform wandless and/or wordless magic, but the wizard must be extraordinarily powerful, not many can. It would make sense if magical spell quality is decreasing that this ability is everywhere in magic users in Merlin. Wands don't even exist in the show, suggesting that they aren't needed and haven't been made. Merlin might have had something to do with the founding of Hogwarts, or at least influencing it's founders, as he himself has had a lonely childhood because he was the only one able to do magic in his village and had to keep it a secret. He feels for Morgana when it is revealed that she has magical powers for the same reason. It makes sense that he would want to help young people separated from other magical users. This would mean that Merlin (and for that matter, Morganna) is, in fact, muggle-born... an extremely interesting interpretation that many pure-blood families in Harry Potter would be outraged about. There are, also, pure-bloods in Merlin, the druids, for example, of which Mordred is a member. This era could very easily be the reason that there is hatred towards muggles, and by extension muggle-borns, in older pure blood families given Uther's hunting down of magic users.
 * I've thourght about this too, and do kind of make sense... Only problem is, that in Harry Potter it is said that Merlin went to Hogwarts and was sorted into Slytherin... (Source: Pottermore)
 * Time messes things about. Could be a mistake.

Lancelot will be back at some point
Less a WMG, more a foregone conclusion.

There is Hammerspace cloning facility underneath the castle
...how else do you explain the endless supply Red Shirt knights that keep popping up after getting slaughtered? I mean, there's an entirely new flock next episode. Surely Camelot is running out of noble sons by now.
 * Chuckle, snort, you know what they say about nobles and making sons...

Merlin is actually set in the future
The characters are actually reincarnations of the legendary figures rather than the legendary figures themselves, hence there are parallels, but it's not quite the same. The world they are living in is a few centuries down the line from a devastating nuclear war, after which the survivors turned away from technology. That's why Gaius' medical knowledge seems too advanced, and why they have potatoes and tomatoes and sandwiches, and why all the other anachronisms. Also, at some point during the war a weapon was created that was powerful enough to punch a hole in the fabric of reality, letting in the magic (and the dragons, and so on) from the world of the Sidhe. This is why there are still portals, like the lake. There are also those strange and creepy monsters which are quite possibly genetic mutations.
 * I'm actually kind of hoping this will be true.
 * And the ancient prophecies that the Dragon keeps talking about are the original Arthurian myths.
 * ...Which means that the future is not set in stone after all.
 * Oooh, oh, I like this. The dragon is actually an AI that's gone insane over the last few centuries and has become obsessed with Arthurian lore, and is trying to manipulate everyone into fitting it as best it can. It also explains how it can survive chained up in a cave where there certainly must be a lack of food. OH! And how they have pagan festivals and talk about gods but also refer to the "old religion" as no longer being present in the same breath. And have such relatively advanced medicine/science (or at least attitudes towards such) and don't mention the Romans at all.
 * Uh, maybe Kilgarrah's an AI, but the pagan stuff? That's the people of the Old Religion keeping it alive in secret. It happened in the real world. Most Catholic traditions especially are derived from Paganisim.
 * Not much of a secret when it's an official and public celebration. And if this was The Other Wiki, the second statement would have a big ol' "citation needed" slapped on it.
 * Catholicism is very, very different from paganism, though some of the traditions (like holidays) were placed or handled to either "baptize" pagan traditions or to hide from pagans. However, as part of this whole war theory, paganism could have come back and contested with Christianity again... Because while Merlin references the gods, Arthur and Hunith both mention "God" (episode 1) or "Lord" (episode 1 again).
 * The numerous monsters like the Griffin and the Questing Beast are actually animals mutated by the nuclear war. Magicians are also mutants, which explains Morgana's X-Men style discovery of her powers.
 * Magic isn't caused by a hole in reality, but is just a form of radiation. While most humans simply developed immunity, a few became true mutants and could manipulate it. Sidhe are the original, more horrifying mutations of humans.

That was not Igraine.
The whole thing was just too damn convenient And also... earlier

In Albion, time and space are relative
That's why the show is is Anachronism Stew, and also why, for example, Merlin managed to catch up with Uther and Morgan in To Kill the King, even though he was on foot and they were on horseback. And other such things. Everyone is aware of this, but it's normal for them, so they just don't talk about it.

The Druids are time travelers.
Or some form of it. Mordred was the child of Morgana and Arthur, and after some event caused her to turn evil, she sent him to study with the Druids. They sent him through time to meet Merlin for whatever reason (or perhaps meet the man he would one day kill). It would explain why there are texts written about him, and how Mordred appeared, and Morgana's strong connection with him.

The Dragon is Merlin's father
How many people seriously thought he was going to say 'I am your father' in "Le Morte D'Arthur"? Also, Hunith's comment about Merlin and Arthur being two sides of the same coin would imply that they've met... (Obviously, this is assuming that the dragon can shapeshift into a human).
 * If the Dragon could shapeshift into a human, then surely he could have slipped his chain and escaped long ago?
 * The chain could have a spell on it to stop him from shape shifting. It's a tiny little thing compared to the dragon, and surely he could have broken it if it were just a chain; but chains in fantasy aren't always mere material chains.
 * Jossed by the end of series two. But it would appear that the Dragon is indeed bound by magical chains.

Merlin's father is one of the Sidhe
This would explain both his talent for magic and the way he knew how to use the Sidhe staff straight away in The Gates of Avalon.

Also, in The Gates of Avalon, Merlin has to slow down time to be able to see the Sidhe properly; but Aulfric seems to be at normal speed. Assuming that he, in human form, would have the same problem that Merlin did initially, he may also have slowed time to speak with them. This suggests that it's a Sidhe ability.

Merlin is The Doctor
According to Doctor Who continuity, they're the same person.
 * Wouldn't the episode 'Midnight' be some sort of paradox, then?
 * Only if Merlin and Jethro are connected... it could just be a really uncanny resemblance. Maybe that's why the Doctor warms to Jethro - 'oh, you remind me of me when I was a boy!'.
 * Jethro really isn't like Merlin, though. They look alike, and yes, Jethro seemed to have a strong moral compass, but he was pretty much an Emo Teen. Merlin, on the other hand, is usually cheerful and easygoing. And then, there was an inverse of how they handle lifeforms/magical creatures. Merlin is usually cautious and little terse. At first, Jethro was actually somewhat cheerful. ('My name's Jethro!' or 'I'm Jethro!' Something along those lines; the point is, he sounded fairly friendly when introducing himself to the lifeform)
 * Nope, no paradox; identical doubles pop up everywhere in the Doctor Who universe. The Doctor's already got at least two, not counting the alternate Tenth Doctor.
 * One way this could be, the all prevalent WMG Time Lord factor: the Chameleon Arch. Merlin's mother was a companion of the 11th Doctor post Amy and Rory, a visit to the "just barely pre-Arthurian era" resulted in her falling in love with Merlin's father and staying behind. Than the Doctor regenerated into someone with not only a very strong resemblance to to a one-time acquaintance...only much younger. An attempt to continue as usual resulted in a mortal injury, soon enough after his regeneration to cause all sorts of instability. Fearing the beginnings of the Valeyard, the Doctor recorded a message for his now-former companion and (he assumed) her husband, asking that they raise him with no knowledge of who he really is, pointed the TARDIS at her home village, and chameleon arched himself, only with a set DNA pattern to replicate that of the companion and "husband". Only he didn't know that Merlin's mother's love had been forced to desert her. Still, when he arrived, she hid his TARDIS and his fob watch and raised him as her son, until the DNA programed from her lover began to change him, causing him to be magical. So begins the plot of the Merlin series. Eventually, Merlin will be fatally wounded, with his mother present. She opens his fob watch, and he regenerates into a MUCH older form, but due to current emergency he sticks around to help Arthur, thus accounting for the modern image of a young Arthur helped by an elderly Merlin despite the original seeming closeness in Age.
 * Or: Merlin is Captain Jack's grandson, Hunith being his daughter, born of his time-traveling shenanigans. It would explain the blue eyes, and she was probably quite good looking when she was younger, the cares of being a single mother to a magical son and a peasant eroding her looks). It would explain Merlin's ability to stop time and his ridiculous resilience (come on, he should be dead several times over by now, considering how much he gets clobbered).

Morgause is the daughter of Gorlois and Igraine or Uther and Gorlois' wife

 * That is how it was in the myth.
 * Morgause had a strong connection with the spirit of Igraine and this would explain Uther's anger at Morgause being alive.
 * But wasn't Igraine barren in this version of the myth? Hence Arthur's being "born of magic".
 * Complications with a pregnancy can sometimes render a woman subsequently barren.
 * Arthur born of Magic: TV - Ygraine was barren and the pregnancy spell cost her life. Myth - Merlin cast a spell to make Uther look like Gorlois. This could not have happened in TV version because Merlin is too young.
 * Well, how about this? Uther and Ygraine keep trying for a child, but it doesn't work. Then, suddenly, she's pregnant. Uther is overjoyed, but then it comes out that the child is actually Gorlois's. But Uther is in one of his nice moods, and decides to forgive Ygraine and Gorlois, and let the child (Morgause) live. However, this also gives Uther the idea of using magic to conceive his and Ygraine's child, now that he sees the possibility is there. After Ygraine dies, Uther 'takes it out on' magic users, and also wants to have Morgause killed, for giving him the idea in the first place, but she's smuggled out. Grieving and looking for comfort, Uther also sleeps with Gorlois's wife, begetting Morgana. (This accounts for the Morgause-Ygraine connection, and while it does make Morgause and Morgana not actually sisters, it still makes sense that they could consider each other such).
 * Ygraine is a serial adulteress and a forger.
 * Myth - Ygraine married Gorlois and begat Morgause and Morgana. then she married Uther and begat Arthur. Later, Arthur and Morgause begat Mordred. Film conflated Morgana and Morgause into one character. TV - Ygraine had an affair with Uther; Morgana is Uther's biological daughter.
 * Uther, Morgause, Morgana, Cenred fit the trope that named, recurring Aristocrats Are Evil.
 * Contrariwise Merlin, Caius, Gwen, Gawain, Lancelot are good, named, recurring Peasants.
 * Merlin forged a patent of Nobility so that Lancelot could compete in the tournament.
 * Arthur is good. Ygraine is Saint Lily Potter.
 * Therefore, Ygraine had an affair with some Peasant to beget Arthur. Ygraine was herself a Peasant with a forged patent.

Mordred is the son of Arthur and Morgana.
Both Morgana and Arthur are old enough, and there was some...tension between them in season one. Mordred also has Arthur's blue eyes and Morgana's brown hair. Morgana's pregnancy was covered up by Qwen and Arthur to preserve her from gossip. They did not name the baby and gave it to some passing druids. The druids gave it to Cerdan, who raised him as his own. This is why Morgana and Arthur did not recognize Cerdan and Mordred when they first met. Also, both Morgana and Mordred seem to have some sort of telepathic link. Its because they are son and mother! Its where he inherited his magic from! Morgana and Arthur hid their...fling away and stopped their relationship.

This was also what happened in the original legends. Except Morgana and Arthur were half-siblings.
 * No. This was what happened in the movie. Excalibur conflated Morgana and Morgause into one character because they only had 2 hours to tell the whole story and because how many evil sisters does a king need?
 * Alternatively, Mordred is is the son of Morgana and someone else but not Arthur.
 * Like pretty blue-eyed sensitive boy Merlin?
 * But Mordred is much too old. We don't know how long the episodes are apart but none of them have visibly aged.
 * This troper was told the version that Mordred was Morgause's son. Being in America, I'm only up to season two, but so far it seems possible.
 * If this turned out to be the case on the show I would let loose such a squeal of fangirly joy that people in the near vicinity would surely go deaf.

Anhora wasn't lying about one of the goblets containing poison
One of the goblets did contain poison, which could only be neutralized by the contents of the other goblet. Part of the Secret Test of Character was that Arthur had to be willing beyond a doubt to die for Merlin.
 * That would the fix the plot hole entitled "What if both of them drank from one goblet and neither died?"

Arthur will eventually find out that Merlin is magical
Arthur's response will be to spare Merlin by having him leave Camelot. This will lead to a convenient Time Skip wherein Arthur will assume the throne while Merlin will become a wandering magic user to learn more about magic and the Old Religion. When Merlin and Arthur are finally reunited, Merlin will be a much more powerful and competent magician and bring with him a very familiar sword...

Gone, gone the form of man
we might get to see Merlin and her set up a few problems for each other. Perhaps one of these will revolve around a newly introduced young knight of Camelot with red hair called Jason Blood. Ok, it'll never happen, but it would be cool to see.

Morgana will be the one to seal Merlin away
The series seems to be setting her up as Merlin's evil love interest which will cause them to have mixed feelings about being on opposite sides. She will pretend to go along with Merlin and Arthur, but will seduce and betray Merlin by learning his secrets and sealing him away into a tree with one of his own spells. The result will be that Merlin won't be at Arthur's side during the final battle and Mordred will successfully kill Arthur, leading to a Downer Ending for the series.

Morgana is dead.
The poison actually did kill her. Morgause, not wanting to lose her sister, made a magical Deal With the Devil to bring her Back From the Dead. Only, this did not bring her back but in fact invited a malevolent spirit to inhabit her body. Morgause does not know this, and 'Morgana' is using her along with everyone else for some nefarious plan (besides the obvious of killing Merlin and Co.).

This could explain how she became pure evil and with a desire for revenge so quickly. Nimueh, maybe?

Morgana will be forced to flee Camelot in the upcoming second part of The Tears of Uther Pendragon.
There's absolutely no way she will be able to continue her masquerade after Merlin inevitably foils her and Morgause's plan. Everyone, including Uther, will know that she is a sorceress and how far she has fallen. Uther, being the complete Jerkass that he is, will probably remark that he should have known and killed her years ago.

Arthur's feelings about magic will be set back yet again because of these events.

She will escape with her sister from Camelot, and continue to be a menace in the future. More than likely she will show up for the finale as the Big Bad.



Arthur has already figured out that Merlin is a wizard.
Of course he won't say anything, since he's now in on the secret. Not only has he been in the same room as Merlin using magic about a thousand times, he always seems a little too dismissive and even angry whenever Merlin is suspected of being a magic user. (See "The Mark of Nimueh" and "The Witchfinder.") Compare this to his resigned acceptance that someone like Gwen may have used magic to cure her father. Arthur is fully aware Merlin has saved his life on numerous occasions, which explains why he insists on Merlin being by his side for any dangerous journeys or battles, even though he's usually loath to put other people's lives at risk.

The series will end with a Kill'Em All, utter Bittersweet Ending / Downer Ending that will go against everything the legends say.
Face it, this is a BBC series and already some things have been very subverted or outright changed from the original stories (some people have even already died who shouldn't have).

Something like the following may happen:

Arthur will find out about Merlin's magic (probably due to it having to be used in front of him to prevent his death) and will understand and help him due to all the good he has done, but he will be forced to flee from Camelot as Uther would probably quickly find out. After this, everything will immediately go downhill and the Big Bad Duumvirate of Mordred, Morgause and Morgana will attack Camelot in full force and without mercy.

Merlin will rush back to find most of Camelot in ruins and nearly everyone slaughtered, including Gaius, Gwen and Uther. Morgana will hold a critically-wounded Arthur hostage and taunt Merlin, who will decide to take none of that crap and instantly kill Morgana with some spell before she can do anything else (since she is the most inexperienced of the three, she wouldn't be able to defend as well). Arthur will live long enough to say farewell to Merlin and for being a good and loyal friend, before dying.

Merlin, in both anguish and rage, will find Mordred and Morgause and ruthlessly kill them both in an epic magic battle far beyond anything seen so far, which will exhaust most of his magic power.

The Great Dragon will then appear, and instead of taunting or trying to attack Merlin will congratulate him on fulfilling his destiny, albeit much earlier and in a different way than he expected. Merlin will be outraged.

The Dragon then explains that, with Uther dead and Camelot destroyed, and with the 'Great Three' magicians dead, magic will indeed return to the land but in a much different and less visible form (technology). Regular magic will diminish to the point where it will be gone forever. Also, with this accomplished, he reveals that his time in the world is up and tells Merlin that he will be the one to unite Albion and not Arthur. When Merlin complains that he said multiple times that Arthur was to be king, the Dragon laughs and says that prophecies can be interpreted many different ways, before flying away never to be seen again.

Merlin then meets with Geoffrey of Monmouth, one of the few survivors of Camelot, and explains everything to him. They agree that Arthur is needed else Albion will fall into chaos, so Merlin uses a spell to permanently switch his appearance with Arthur's body (using the rest of his magic and forever losing the ability to use it in the process), meaning that Merlin was the one 'killed' by Morgana instead.

Due to the 'miracle' of Arthur surviving, the survivors gather together and crown him their king. Eventually, Camelot is rebuilt bigger and better than before, and peace and prosperity reign throughout the land. Albion is indeed united. The ban on magic is lifted, but already magic is fading and very few people can do much with it.

Lancelot, having not been present in Camelot, will return and become 'Arthur's' most loyal knight and friend. Besides Geoffrey, only Lancelot will know the truth about Merlin.

Merlin, however, is still deeply saddened and bitter over where his destiny led. Geoffrey, in secret, chronicles the legend as he thinks it should have rightfully happened, making the legends of King Arthur we have a very early form of Fanfic.

Merlin lives to an old age, but never marries. His heir turns out to be an historical figure, who takes over the land and renames it, making Camelot fade into legend and myth. One night, he awakens and sees Freya in his room and realizes that he is younger and back in his own body, meaning he has died. As he is ready to leave with her he also sees Arthur, Gwen, Gaius and many others who stay silent but smile at him. Then the screen fades to white.

And the fandom will cry.

Freya was deliberately invented to Ship Sink Merlin and Morgana.
It didn't work, however. Every other attempt by the writers since then to do so hasn't either.


 * It also could have been to Ship Sink Merlin/Arthur.

Episode 5 of Series 3 will be either a test, All Just a Dream, or a time-travel episode.
According to the press release, Merlin will visit a wizard who shows him a vision of Morgana murdering Uther. Merlin will try to prevent this with 'fatal consequences', which means that he will probably confront Morgana.

What could happen:

She will try to kill him with magic but Arthur will come out of nowhere and be hit by it, dying instantly. As a result of this, everything will quickly go downhill. Morgana will blame Merlin for his death in front of everyone and accuse him of sorcery (also having bruised herself to look as if Merlin hurt her), and Uther will likely try to run Merlin through right then and there. With no way to defend himself he will use magic in front of him to escape, further proving Morgana's claims to Camelot.

With Arthur and Merlin gone, Morgana will proceed to murder Uther in the very fashion that was predicted, and claim that Merlin came back and did it. With both Uther and Arthur dead, she will be crowned as Queen and lift the ban on magic. Her reign would be far from ideal though, instead being as much of a tyrant as Uther if not more so, and the land would suffer.

Merlin, having failed his destiny and having nowhere else to go (the Dragon, if he appears at all, would reject him), will turn himself in to be put to death, completely and utterly broken. Morgana and possibly Morgause will torture him to near the point of death, and one of the following things would later happen:

a) Merlin will end up back in the cave with the wizard, with it being revealed to him that it was all a test, and to warn him what will happen if he should fail.

b) Merlin will wake up in his bed, all of it having been a nightmare. He tells Gaius, and he tells Merlin that the wizard (Taliesin) died a long time ago and that he must have been reading about him. Merlin will reveal that he never heard of him before, meaning that he was in fact sort-of Dreaming of Things to Come, warning him what will happen if he should fail.

c) Gaius, or someone else, will sneak in and give Merlin a magic trinket to go back in time. The cost is that the person will soon die after using it, as it is powered by life force. Merlin will change his appearance with magic, use the device, and run across his past self before he confronts Morgana. He will warn him not to do what he is planning, and then die.


 * Jossed. The events of the episode were entirely Canon.

Uther will be Killed Off for Real by Morgana, having usurped the throne of Camelot for herself and forcing Merlin, Arthur, Gwen and others on the run for their lives.
Promotional pictures and videos for later Series 3 episodes heavily suggest this may indeed happen. Most of Camelot probably hate Uther like no other and will rally support behind her, and those who don't will be executed or forced to flee.

Morgana, wearing the crown, in shown to be ordering an execution. This is probably Uther Pendragon, being killed in one of the same ways he had done to many others, as an act of revenge and irony.

Gwaine, Lancelot, Leon and some other unnamed knights are shown to be loyalists to Arthur, and likely will become the first of the Knights Of The Round Table.

The later episodes of Series 3 will be one Wham! Episode after another, with large heaping doses of Nothing Is the Same Anymore, and Status Quo Is God being lifted once and for all. Merlin's magic will be revealed to everyone, Uther will die at Morgana's hand with Merlin helpless or too far away to prevent it, and the last episode will end with Morgana being defeated and driven away from Camelot, since she must survive in order to later align with Mordred. Morgause has no such prophecy protecting her and may well die, probably right in front of Morgana at Merlin's hand, which will cement her rage and hatred for him forever. Arthur will be crowned King and Gwen his Queen, and all will be well in Camelot for the time being.

Doesn't that sound like a Grand Finale, if it ends up being true? Perhaps Series 3 of Merlin is going to be the last, or for whatever reason the writers are deciding to change the show for good.



Morgause has Confessed Morgana.
Or some corresponding form of mind-control magic. Perhaps the blood link helps.

In any case, since Morgana has returned, she has demonstrated absolutely no qualms about killing off her former friends. In "The Castle of Fyrien",

Another indication is her lack of concern for the well-being of Camelot's citizens. When she kills a guard who spots her leaving, it almost seems like she is relishing it. In short, her actions have become Card-Carryingly Villainous, which is completely out of character for her.

Perhaps, then, she is quite literally out of character? Perhaps Morgause has enslaved her will? It's not implausible that she does have that (or similar) magic, since if she did, it could take a year to cast on someone, which is why it's taken so long to get Morgana back into Camelot - she was Confessing her the whole time. Then there would be a simple explanation as to why she doesn't just throw it around, using it on everyone.

So Morgause sends the mind-enslaved Morgana, who now wants only to please her mistress back into Camelot with orders to act normal until she's needed. And to smirk about this situation EVERY SECOND THAT NOBODY'S LOOKING.


 * Bad writing. That is all. Plus Katie McGrath can't act evil particularly well, which makes it seem as if she would fit in well with the likes of Cobra (replace the shouts of "Cobra!" with the smirk, they have nearly the same effect).

Morgana was not meant to be evil, and may return to good
The Dragon wanted to destroy Camelot. But what was in his way? Well, Morgana could not only predict his actions (a general's wet dream), but tell Merlin he was the Dragonlord. So the Dragon ordered Merlin to betray her. After years of Uther's treatment, Morgana is half a step away from C-PTSD (PTSD resulting from multiple traumas instead of just one). When Merlin betrays her, it's the straw breaking the camel's back, and Morgana has a psychotic break. Morgause, seeing this, brainwashes Morgana into being a Card-Carrying Villain. However, there is a flip side to this. Because Morgana is reacting to trauma, Arthur should, in theory, be able to reverse it--if he could confront her, she could become good again. And if this happened, Camelot could completely change...

Princess Elena is the Elaine that Lancelot marries in the myth
How adorable would it be if he married her?

Uther has an alcohol abuse problem.
Go back and watch through all the times Uther has an outburst of rage. Now, what was he doing just before? Have there ever been any times Uther did something...Moral Event Horizon-ey without having had a goblet in his hand a moment before? And the decisions where he didn't he's never shown any signs of regretting, as opposed to the ones where he did. Also, what is Uther most often seen doing when Camelot is in crisis?

"Dragoon" will become Merlin's sorcerer alter-ego
Since Merlin is based off of Smallville, future episodes may have more of Clark Kenting twist involving Merlin. Dragoon is already notorious as a twisted sorcerer, so Merlin has a persona to do the dirty work that he would normally get in trouble for doing. It would also parallel nicely with the legends surrounding the legendary Merlin who could switch from a young boy to an old man.
 * Called it!

Merlin will someday learn to shapeshift into animals
In the original myths, Merlin does in fact fight a duel through shapeshifting (That duel with Mim wasn't just a Disney thing). Also, Merlin's name is that of a bird of prey, and I can't be the only one who would love to see him turn into a bird because of that.

Lancelot will become Galahad's Big Brother Mentor
Obviously Galahad will have to come in sometime soon to join the knights since he's such an iconic part of the original myth, but he can't possibly be Lancelot's son. It would be sly way to reference the fact that Galahad in the original Arthurian myth was Lancelot's son.

Freya and Merlin will become a Battle Couple
Freya seems to be learning how to control her powers as a spirit of the lake and she and Merlin still care about each other. Also, since Morgana's gone, there has to be another woman join the main cast to avert the Smurfette Principle. Merlin's power is more fire-based, being a Dragonlord, so Freya could support him with water powers.

Madame Mim will show up
It will be part Mythology Gag and part homage to Disney's The Sword in The Stone.

Gaius has been protecting Uther since the beginning of the Great Purge.
Are we really supposed to believe that no evil magic users tried to attack Uther in the twenty years before Merlin came to Camelot? And if they did, how is Uther not already dead? It could be that Gaius has spent those twenty years doing what is now Merlin's job, running around and stopping these evil-doers behind Uther's back. No doubt it came as a relief to the old boy when a young helper turned up to take over.

The Fourth Season will include Merlin's version of Kate.
The writers know that they need to include a new female character to avert the Smurfette Principle, but they will make her a poor man's Morgana who everyone inexplicably likes and who will become Merlin's Designated Love Interest.
 * Oh please no. The only good thing about Kate was the knowledge that there will never be anything as bad as Kate ever again.
 * Oh God. I can live with Gwen being a Smurfette. Please, let her stay a Smurfette.
 * Hopefully they will avoid the Smurfette Principle, but they'll do so by bringing back another important female character who already exists, like Hunith, Freya, Vivian, Elena or Alice.

This Merlin is the same Merlin in Harry Potter
The whole story takes place hundreds of years before Hogwarts was actually established.

Assuming the above is true...
The castle of Camelot will turn into Hogwarts in the future and will become the academy for magic we all know and love.

The Love Triangle
I'm calling it now: the scene that we saw in the season four trailer, is a magical set-up. Either Morgana is sending Arthur bad dreams, Morgana is sending Lancelot bad dreams, or the Guinevere we see in that scene is not Guinevere at all. It's Morgana, disguised as Gwen, and trying to stir up trouble amongst Arthur and the knights. Which begs the question: where is the real Guinevere?
 * Or Lancelot could be a fake... Or even a dead Lancelot with some kind of demon possessing him, which is later defeated... And that's how we get to the possible spoiler
 * Or the trailer makes it seem like a bigger deal than it is. Gwen is already shown to have a tendency to express her joy at finding out someone she thought dead is alive by kissing him (right, Merlin?), and Arthur has shown a tendency to overreact regarding any affection Gwen shows to another man (seeing Gawaine, who was never remotely serious competition for her affections, saying goodbye to her had him getting upset about how they seemed "awfully friendly"). Combine the two's tendencies with Lancelot returning from the dead and Arthur walking in at the worst moment, and you get a setup for either hilarity or disaster.
 * Or the trailer makes it seem like a bigger deal than it is. Gwen is already shown to have a tendency to express her joy at finding out someone she thought dead is alive by kissing him (right, Merlin?), and Arthur has shown a tendency to overreact regarding any affection Gwen shows to another man (seeing Gawaine, who was never remotely serious competition for her affections, saying goodbye to her had him getting upset about how they seemed "awfully friendly"). Combine the two's tendencies with Lancelot returning from the dead and Arthur walking in at the worst moment, and you get a setup for either hilarity or disaster.

Gilly is Galahad
He's brave, pure of heart, and has magical powers to aid him. He might not be the complete Marty Stu Galahad from the original legend, but has the potential to nudge Arthur in the right direction regarding the nature of magic by dying as a knight of Camelot after protecting Arthur.

Mordred is the son of Morgause
Mentioned earlier on the page, but it has enough merit to be recognized. Morgause appears older than Morgana, enough that having a child around his age is not out of the realm of possibility, and we really have no idea what she's been up to before showing up in Camelot. She might have given up the child to his Druid father or, due to prophecies, left him behind with supporters of the Old Religion. In "The Once and Future King", it is Morgause who is Mordred's mother, and Morgana being his aunt could explain their connection.

Morgana's mother's name
Uther mentioned in 3x05 that Morgana's mother's name was Vivian (or some variation). Given that the show has already one character with the same name (Lady Vivian from 2x10), it's possible that they gave the mother that name because she will be the Vivian from the legend. But she was the Lady of the Lake, and that role has already been fulfilled by Freya...

Merlin is immortal
I'm not sure how much of a WMG this is, since it's backed up by myth, but it needs saying. Evidence: Edwin couldn't kill him, the Cup of Life wouldn't take his, he survived the Dorocha touching him when it was explicitly stated that no mortal man could, and the Callieach.
 * Alternatively, it may not be a matter of being unable to kill Merlin, so much as making it stick. In "The Poisoned Chalice", . And in "To Kill The King" and "Le Morte d' Arthur",.

Everything is shown from Merlin's point of view, and Merlin has Black and White Insanity
It explains all the Moral Dissonance.

Old!Merlin will become Morgana's mentor and Parental Substitute.

 * In Morgana's prophetic dream, she seems to expect Emrys to help her so she probably does not consider him an enemy. Morgana hates Uther and never knew Gorlois, so she'd probably gravitate towards a father figure.

While recovering from her poisoning, Morgana dreamt about the future...

 * ...and saw Gwen, Arthur and Merlin trying to defeat her and generally hating her guts. This soured her against her friends and resulted in her rather sharp Face Heel Turn.

The upcoming episode Lamia will parody the common Mary Sue

 * A beautiful damsel in distress inexplicably causes all the knights to fall in love with her and become uncharacteristically jerky towards each other, especially Merlin? Sounds like the classic Damsel Scrappy of a Jerk Sue with a side of unholy demon brought into the mix!

Merlin will save Camelot with an army of magic users

 * Merlin's magic will be revealed and he will be banished. But he will not have to be alone because he has the help of two dragons, the Druids, and the Carthars. So Camelot will be under attack and all hope will seem lost. Arthur and his knights will prepare to make their final stand. But then Merlin swoops in on dragonback and all the friends that Merlin made during his time in Camelot will arrive at his back to help defend Camelot.

Merlin's secret will be revealed in the Series 4 finale

 * Almost every episode of season 4 seems to show some sort of betrayal and Arthur's reaction to it. Seeing how much Arthur was crushed by, putting him in an already vulnerable place where Merlin is the only person he can depend on. With all the emphasis on betrayal and the Merlin/Arthur friendship, all of it building up to a finale reveal would be the most likely conclusion.
 * Jossed.

The series Merlin will end on a high note

 * Arthur will finally learn about and accept Merlin's magic, Arthur and Gwen will be  married, the Knights will come together at the Round Table, and the series will end with all of them bantering with each other and riding off to Camelot to found Albion together.
 * One down, three to go.

Merlin is a Dragonborn.
Seriously, it all fits. He can speak the language of Dragons, and has demonstrated usage of Slow Time (TIID KLO UL) and Unrelenting Force (FUS RO DAH). He's also the chosen one. However, in this 'verse Dragons aren't quite so bad.

Aithusa was sent by Mordred to heal Morgana
Kilgharrah, the Great Dragon, has said that Morgana and Mordred will form an alliance in the future. We do not have much of a back story on Mordred(as far as the show itself is concerned and we know how the show diverts from the original material), so for all we know he could have the ability to control a young dragon like Aithusa. Assuming Aithusa is not evil, nor naive of Morgana's cruelty, Mordred could have felt Morgana in danger, could not teleport to her, and was too far to reach her by foot, so he sent Aithusa to heal her. He could be a Dragonlord, or he could just be a powerful warlock that could control the young dragon.

Arthur isn't King Arthur
His son is. Series 5 will establish that Gwen is pregnant early on. The rest of the series plays out as you'd expect up to the finale, during which Morgana attempts to conquer Camelot (as per usual). Gwen goes into labor early on in the battle. Arthur assigns Merlin to look after her. The battle carries on, and Arthur and Morgana battle it out, eventually winding up in Arthur and Gwen's chambers, where Gwen has just given birth to a baby boy. Merlin manages to defeat Morgana, but Arthur is mortally wounded and dies in Merlin's and Gwen's arms. Gwen names her son after his father and tells Merlin to sneak him out of Camelot and have him raised away from the dangers of royalty. Gwen loses the will to live and dies shortly after. Merlin obeys Gwen's dying wish and sends the infant Arthur to live with a kind family of peasants before taking Excalibur and returning it to the stone. He then returns to Camelot, keeping the kingdom together despite its lack of ruler. Flash Forward twenty years, when a young man who looks uncannily like Arthur approaches Camelot from afar. Kilghara's voice then narrates: No young man, no matter how great can know his destiny...

Uther's Great Purge actually made magic stronger.
Hence why Camelot was able to wipe out the vast majority of magic users and dragons, yet can't do a thing about it in the present age. A big part of the Old Religion is balance, so with magic so widely distributed, it was a lot weaker. There's also a lot of instances in show where non-Merlin magic users do things that are incredibly strong magic rather easily. One case is Servant of Two Masters, where Gaius mentioned that it would take a high amount of skill to apply a Fomorrah to a person, and yet all evidence seems to point towards Morgana doing this on a whim rather easily. Another is, while they needed magic to get the Great Dragon to Camelot, they were able to capture it the last time, and when it got out twenty years later they couldn't even fight it. It also might explain why Merlin and Morgana are so strong magically: they were born in the midst of the Purge. Nice Job Fixing It, Villain.