Christopher Eccleston has been attached to the Thor sequel. The former Doctor Who actor is in final negotiations to play a villain in the upcoming sequel, according to The Hollywood Reporter. Eccleston will star as Malekith the Accursed, the ruler of the Dark Elves of Svartalfheim, if a deal is concluded. The Thor sequel, to be directed by Alan Taylor, is officially titled Thor: The Dark World and features the original cast. (source)

Where are Hela and Enchantress in the Thor films though?

I need to see the two baddest bitches of Hel and Asgard.