Thanos cld easily fit the beyonder type role if necessary to change Alrim to Kurse.
I do expect more supervillains in the movie not just kurse.My moneys on Hela not surtur. Surtur is best reserved for a movie that focuses on the Twilight sword/Ragnarok saga.
Thor 1 started a journey-Thors journey to becoming a worthy king of Asgard and by extension a worthy ruler of the 9 realms.
Logic dictates that during his journey Thor will have conflicts within the 9 realms-in otherwords during this journey he will only be facing 9 realm villains.The ultimate 9 realm villain is Surtur so hes best reserved for last.I expect Thors journey to becoming will be concluded by Thor 3,leading to a new trilogy with Thor dealing with threats from outside the 9 realms.