5. KARNILLA
In a deleted scene from AOU we saw Thor be possessed by a Norn. Since the Norns are responsible for the fate of Asgard it would be logical that there queen would make an appearance at ragnarok. The Norn Stones could also be the vessels of the Soul stone.
4. SURTUR
Surtur is the ultimate enemy of Asgard and its prophecied destroyer. It has been stated that Thor 3 will take place on a new planet, this could infact be Korbin, the homeworld of Beta Ray Bill which Surtur destroys to forge the Twilight Sword.
3. MANGOG
Mangog is a creature created from the souls and hate of the Vanir, a race of gods destroyed by Odin. Mangog could be created using the Soul Stone in the film, and be a pawn of Loki or Thanos. Mangog is also one of the only villains who could beat Thor and Hulk by himself.
2. THE ENCHANTRESS AND SKURGE
The Enchantress and Skurge help Loki in many of his schemes and would be a great villanous duo to fight Thor and Hulk. It has been rumoured that Marvel will be introducing a female villain in Thor 3 and the Enchantress would be a great choice.
1. HELA
Hela is the most likely villain to appear in Thor 3. Appart from being the norse godess of Hel she could double as Thanos love , mistress Death in the mcu. Since the Soul Stone will likely be appearing in the film it would be logical for Hela to wield its power to steal souls.
I hope this has been informative; please post your thoughts.