Believe it or not, I'm an Aquaman fan. Especially with the relaunch of the DC Universe. DCnU Aquaman is badass and reading the stories make me want a film even more. I've had a few ideas about casting and some about story as well albeit not a full plot as of yet. My film would also be a part of a unified cinematic universe.
I would start it with a series of flashbacks, firstly, Tom Curry "saving" Atlanna who he thinks is drowning. This would lead to Tom waking up the next morning with Atlanna gone, it would be heavily implied that they had slept together. The next flashback would show Atlanna returning with a baby and leaving him in Mercy Reef for Tom to find and raise. The next flashback would be 10 year old Arthur asking his father why he's "different", which cuts to Arthur breathing under water, telepathically communicating with marine life. After this it would move to teenage Arthur, grieving his father's death and inadvertently causing Manta's father's death. Then it would cut to the present day, where Arthur is visited by Vulko who begs him to return to Atlantis because, as the Atlantean advisor, Vulko doesn't believe Orm, Arthur's half brother is worthy to be king. By this stage Arthur is married to Mera and living with her in Curry Lighthouse. He refuses to return to Atlantis until some Atlanteans begin to attack the surface world. Arthur and Mera counter these attacks, Mera only doing so to support her husband, and Arthur discovers that the dying Atlantean king ordered the attacks as he blames the surface for his poor health (pollution, maybe there's an oil spillage).
This would cause Arthur to return to Atlantis where the king has died or dies soon after Arthur and Mera's arrival, making Arthur the rightful king. The Atlanteans are initially hostile towards their new King who is a stranger to them and refuses to go to war with the surface to avenge their former King's death. Orm supports Arthur's choice but then they hear of another attack on the surface despite Arthur forbidding anyone to do so. Arthur returns to the surface and tries to prevent bloodshed between the humans and Atlanteans (who are severely outnumbered). Then Manta shows up. Meanwhile, in Atlantis, a group of Atlanteans try to abduct Mera who stayed behind but she uses her hydrokinetic powers to break free and seeks refuge with Orm. Vulko tries to speak with her but Orm refuses to let him see her and then tells Mera that it was Vulko who ordered the attacks. Aquaman and Manta fight aggressively until Manta releases a heat beam from his helmet, driving Aquaman back into the ocean and back to Atlantis.
In Atlantis, Vulko gives Arthur the Trident of Neptune to aid him against Manta. This is when Vulko gets taken away by Atlantean guards for ordering the attack on the surface. Orm publicly declares that it was Vulko who defied the King and orders him to be taken to the Aquarium (Atlantean Prison). Arthur intervenes, viewing Vulko as a substitute father figure. Orm begins to rally the Atlanteans against Arthur, claiming that Arthur would rather live on the surface than defend his own people. Instead of taking Arthur and Vulko to prison they are cast out of Atlantis. Mera tries to follow but Orm doesn't allow her to leave. Arthur takes Vulko to Curry Lighthouse where Vulko tries to convince Arthur to return and try to win them over, Arthur struggles to understand why Vulko is still so loyal to them even after being exiled. However, he had already planned on returning for his wife.
In Atlantis, Orm attempts to seduce Mera, stating that she would make a fine queen, standing by his side opposed to Arthur's. When she refuses she gets taken away by Manta-Men who begin to storm Poseidonis, subduing any Atlanteans who disagreed with Vulko and Arthur's exile. Orm announces to Atlantis that he found the man who attacked them on the surface and has struck a deal with him. Black Manta then steps forward. Informing them that "King Orm" has enlisted his help to make Atlantis safer and with his help there won't be any more damage caused to them. Two young Atlanteans, Garth and Tula, realise what is really happening and split up, Tula infiltrates the Atlantean Royal Palace and locates Mera who has already dispatches the guards with her hydrokinesis as Tula arrives. Meanwhile, Garth travels to Curry Lighthouse to inform Arthur of what is happening in Atlantis. When he arrives Arthur is already preparing to return, wielding his Trident.
Mera and Tula start taking out Atlantean guards and Manta-Men as they take any Atlanteans who don't agree with Orm to the Aquarium. Garth and Arthur return to Atlantis. Manta-Men spot them and advance on the pair, Aquaman sets sharks on them, revealing he brought back up as an army of marine life follow him including sharks, whales, squids and octopuses. Aquaman sets the marine life on the rest of the Manta-Men and seeks out Orm, finding Mera and Tula in the Palace with the same agenda. Garth and Tula are told to help the Atlanteans while Arthur and Mera look for Orm. They find Manta instead who claims to have used Orm to find Atlantis and Arthur so he can finally get his revenge.
Elsewhere, Mera finds Orm, who tries to convince her that he was forced to do it by Manta, that it was all Manta's idea, he then attacks Mera when it appears she doesn't believe him. Mera uses hard water constructs to fight Orm until Arthur steps in. Orm is outnumbered and surrenders. Arthur exiles Orm and he and Mera have an official coronation, becoming King Orin and Queen Mera of Atlantis. Aquaman tells the Atlanteans that despite his position he still has ties to the surface and in his absence, his Royal Advisor, Vulko, would be left to rule. It would be implied that Aquaman and Mera made love earlier in the film and Mera tells Arthur that she thinks she's pregnant.
Mid-credits: Exiled and furious, Orm adopts a "costume" and a new persona - the Ocean Master and vows to reclaim what is rightfully his.
Post-credits: Black Manta nurses his wounds when a bald man walks in. "You've made quite the impression 'Black Manta', you went toe-to-toe with 'Aquaman' and live to tell the tale." Black Manta asks who he is, the man is revealed to be Lex Luthor who then recruits Manta into the Legion of Doom.
For Aquaman himself, I would cast Liam Hemsworth (Hunger Games, The Expendables 2), brother to Chris "Thor" Hemsworth. Despite his small role in Hunger Games, I was impressed with his performance and he certainly has the look for Aquaman. For the younger versions I would suggest going with unknown actors.
For Mera, Aquaman's wife with hydrokinetic abilities I would cast Amanda Righetti (Captain America: The First Avenger, Friday the 13th). She was also in Role Models which shows her range as an actress, she is also very attractive with all the assets to pull of a sexy but strong (eventual) Queen.
For Aquaman's main contact from Atlantis - Vulko - I would cast Brendan Gleeson (Harry Potter, 28 Days Later). Gleeson has proven that he can play a father-like mentor figure and this is definitely how I would picture Vulko to be portrayed.
For Aquaman's younger, half-brother I would cast Alexander Ludwig (Race to Witch Mountain, Hunger Games). In Hunger Games he played a very brash character that I could definitely see as a jealous younger brother. I don't think he would necessarily become Ocean Master in the first film but he would be well on his path to doing so by the end.
For Black Manta, Aquaman's nemesis, I would cast Mekhi Phifer (Torchwood, ER). I had some trouble casting this one. Other choices were Michael Ealy and Anthony Mackie but in the end I chose Mekhi Phifer because I thought it was quite an original choice for the character and his performance in Torchwood impressed me.
For Aquaman's father - Tom Curry - I would cast Kevin McKidd (Percy Jackson and the Lightning Thief, Dog Soldiers). McKidd was my original choice for Aquaman but he is approaching 40 so I decided to go for a younger actor but wanted to cast McKidd so I thought he'd be a good choice for Aquaman's father.
For Aquaman's mother - Atlanna - I would cast Cassidy Freeman (Smallville, The Vampire Diaries). It wouldn't be a very big role but I think she'd add emotional depth playing a mother who is forced to abandon her child.
As a bonus, in Atlantis there would be two young Atlanteans who admire Orin and even help him. Those two Atlanteans would be Garth and Tula. For Garth I would cast Josh Peck (Drake & Josh, Red Dawn). He seems to be moving into more serious acting but could also add some comic relief. He wouldn't be called Aqualad in the film, simply Garth but he would go on to be Tempest in sequels. I realise he is older than Liam Hemsworth but he looks young and could be made to look younger.
And for Tula I would cast AnnaSophia Robb (The Reaping, Bridge to Terabithia). She has been acting since she was 11 and even as a child she has always given convincing performances in my opinion so I chose her.
For the Lex Luthor post-credits cameo I would cast Jon Hamm (Mad Men, Suckerpunch). Shave his head and you've got one powerful and assertive Lex Luthor.