As the title "The Dark World" reveald on SDCC this year already points to the Dark Elves of Svartalfheim this seems to be very likely now.
However what we don’t know is which intergalactic foe or group of foes The Mighty Thor will be aiming his hammer at. Thanks to a brief chat with Hollywood stuntman James Grogan, we may have our answer. Whilst discussing the importance of acting chops in stunt work, Grogan related the experience to a certain Marvel film that he’s currently training for. “The other day for example, this is quite random but I was working on Thor 2,” he explains, “I had a load of stunt guys in and the director came in and he said ‘Look, we want you all to be elves,’ and I was like ‘What the hell is an elf anyway?’ and he said ‘I want you all to walk around like these supernatural animals,’ honestly it was the most ridiculous thing ever but you just have to not be self aware and get on with it.”
Since the title is Thor: The Dark World it's pretty safe to assume that he is referring to the dark elves. Which would then lead us to believe that their leader, Malekith the Accursed, is one of the main villains. Now we already know that Tom Hiddleston is returning as Loki, and there is a popular story that links him with Malekith. In the comic book Thor #344 Loki and Malekith formed an alliance in order to secure the Casket of Ancient Winters. Perhaps they will use this story arc for the sequel, or perhaps not.
Info about the Dark Elves, Malekith and the possible story arch of The Casket of Ancient Winters
Dark Elves -
The Dark Elves are ruled over by Malekith the Accursed, and the most powerful amongst their ranks was Algrim the Strong. However, Algrim was betrayed by Malekith in a bid to kill Thor. Algrim was dropped into lava and though he survived, he was critically wounded. The Beyonder transformed Algrim into the monstrous Kurse, who seemingly kills Malekith for the betrayal. Years later, Malekith reveals that he survived, and he sets forth an ultimately unsuccessful plot for Hercules to kill Alflyse, the Dark Elf Queen of the Eastern Spires.
The trio of Bitterhand, Wormwood, and Grendell serve Malekith loyally and have attempted to destroy Thor on several occasions, including once while Mjolnir was being reforged
Malekith the Accursed ruled the Dark Elves of Svartalfheim. Malekith mastered sorcery, making him a formidable ally to the other enemies of Asgard. He formed an alliance with the god of mischief Loki on behalf of Surtur. Not long after that agreement, he used a special food provided to him by Hela. The food contained some sort of magic, which gave him control of the humans of Earth. While continuing his quest, he took the life of Eric Willis, guardian of the Casket of Ancient Winters. Before his death Willis conveyed the Casket's location to Loki. Not willing to give up his quest, Malekith Kidnapped the Asgardian Lorelei in order to gain control of the god of Thunder. Thor discovered that he had been tricked into falling for a stack of wood, and became enraged as he learned that the real Lorelei was being held captive by Malekith. Lorelei was eventually freed but, Thor was forced to battle Algrim the Strong. Malekith attempted to destroy both combatants by plunging them into a pit of molten magma; subsequently Malekith retrieved the Casket of Ancient Winters, from the being known as Jagrfelm. Shortly after Jagrfelm's death, Malekith released its power upon Asgard in his quest to rule all nine worlds.
The Casket of Ancient Winters -
An Asgardian relic and the greatest weapon of Malekith the Accursed. When opened, it can reproduce the infinite icy cold of Niflheim. Later entrusted to the care of Edwin Jarvis, butler to the Avengers, by Thor. Causes its holder to grow younger gradually.- Wiki