I was reading a discussion of Merlin, specifically season 5 which should premiere in September/October, and decided to make a list of things that I think will happen in season 5.
1) Tristan becomes a knight.
In the legend, Tristan was a Knight of the Round Table. Since he's been introduced, I think he will be knighted. The writers did the same thing with Percival in the season 3 finale/season 4 premiere.
2) Morgana teams up with King Lot.
Again, another character from legend who has been brought in. He was only mentioned in passing, but Morgana/Morgause were in league with him in the legend. Since both Helios and Agravaine were killed of in the season 4 finale, Morgana will be on the hunt for new allies to help her take Camelot. Again.
3) Gaius dies.
In almost every piece of fiction where there is a strong mentor/mentee relationship, there comes a point where the mentor isn't needed anymore and dies/leaves. Aslan in the Chronicles of Narnia and Dumbledore from Harry Potter are great examples of this. That aside, Gaius almost died twice this season. Since the writers/producers were given the okay for season 4 and season 5 at almost the same time, they could plan ahead a little bit. His near death experiences in this season could very well be foreshadowing for his actual death in season 5.
4) Mordred returns.
Mordred is a central character in the legend. He's Arthur's nephew/son with Morgan Le Fay and grows up to kill him in battle. In the series, Kilgharrah tells Merlin that Mordred's destiny is to kill Arthur. Since we haven't seen/heard from him since season 2, his return is kind of overdue. Whether he teams up with Morgana or not is something we have to wait and see. Since the writers like reinventing the legend, it could go either way.
5) The Round Table appears again.
In the legend, King Arthur has an order of knights known as the Knights of the Round Table. It's an important part of the legend. Some of the knights in this order include Gwaine, Percival, Tristan, and Lancelot - all of whom have made an appearance in the show. The writers even wrote a scene involving a round table into the season 3 finale. The Round Table wasn't mentioned once in season 4, so the writer kind of have to bring it back in season 5. Unless it's mention in season 3 was a one time thing, which would kind of be a cop out on the writers part.
6) Arthur either conquers another kingdom or a foreign kings swears allegiance to him.
According to Kilgharrah, Merlin is supposed to bring magic back and help Arthur unite Albion. The show is set in medieval Britain. Back then, if you wanted to unite two kingdoms, the king/prince either married the other king's daughter if he didn't have a son or you conquered the kingdom in question. Another option is that Arthur becomes High King of Albion, which means the other kings/queens swear allegiance to him. The second option is somewhat popular in fanfiction, but not how I think the writer are going to do it. Especially since Arthur and Gwen are now married.
7) Arthur and Gwen find out about the enchanted bracelet.
In episode 9, Lancelot du Lac, Lancelot gives Gwen a bracelet that was enchanted by Morgana. When Gwen is in the dungeons, she takes the bracelet off and throws it at the wall of her cell. It lands in the far corner and is never seen again. What happened to it? I thought that Morgana was going to brag about enchanting Gwen when she and Arthur had their confrontation, but it wasn't mentioned at all. Maybe I'm reading too much into that scene, but they only have 45 minutes and they spent at least a minute and a half on her throwing it away, including when she was looking at it before she took it off. Of course it could be a continuity error on the writers/producers part.
8) The Cup of Life makes another appearance.
It is a very powerful magical object. A magical object that both Merlin and Morgause had the power to use. Since you need to be able to mirror life and death to use it, only the very powerful can use it. Morgana may fall into this category, considering that she is a powerful sorceress and most of big pieces of magic she's done have involved life/death in some way. In the season 3 finale, it's implied that Arthur has the cup placed into the vaults of Camelot. Since it was never mentioned in season 4, it's probably still there.
9) The Druids also make another appearance.
In episode 10, A Herald Of The New Age, Arthur make a promise to treat the Druids differently than he had in the past. The writers have to have at least one episode to show how Arthur keeps/doesn't keep that promise. There is also the reverence that the Druids have for Merlin. Now that Morgana and Arthur know of Emrys in some fashion, that is going to have to play out at some point.
10) Merlin reveals his magic to Arthur.
This is the moment everyone has been waiting for. Merlin has to reveal himself as a sorcerer in order to fulfill his destiny and help Arthur fulfill his. That's not the only reason though. How long can he really keep his magic a secret without Arthur/Gwen/one of the knights figuring it out? At this point in the show Merlin has been in Camelot for over five years. After all that time, I'm starting to have trouble believing that no one has questioned all those convenient moments that helped Arthur and others defend Camelot. The writers are going to have to address that this season.