While the film is nearly 3 years away, the stories behind give us an idea of some new faces we might meet in Thor: Ragnarok.
This one is a certainty as there is no Ragnarok without Surtur. Surtur, an ancient enemy of Asgard and the ruler of the Norse realm of fire, Muspelheim, represents fire, one of two primal extremes in Norse mythology (ice being the other), and his existence predates Odin. While there is no certainty how we'll be introduced to Surtur, it's likely that Loki, who currently sits upon the throne of Asgard impersonating Odin, will have a lot to do with it.
One of the most powerful sorceresses of Asgard, Amora the Enchantress could play several roles in the Ragnarok story. A love interest for both Thor and Loki at different times in the past, Amora, whose sister Lorelei was seen last season on Agents of S.H.I.E.L.D., often uses her powerful magic to manipulate the minds of men. Tom Hiddleston, whose Loki will be a central figure in Thor: Ragnarok, has made public his desire to have Amora join the ranks of the Asgardians and, after having been considered in the past before being passed over, it's easy to see how she might just turn up this time.
Skurge, the Executioner
If we see Amora in Thor: Ragnarok, it's hard to imagine we won't also see her loyal bodyguard, Skurge. The son of a frost giant and a goddess, Skurge is a massive and powerful physical threat who wields the enchanted Bloodaxe, a weapon of enormous mystical power. Depending on how things fall, Skurge could make an intimidating physical threat and a distraction for Thor while Loki works with Surtur behind the scenes.
The ruler of the Norse realms of Hel and Niflheim, Hela is a goddess with the power of life and death over all, including the Asgardian gods. In the comics, Thor has made deals with Hela to return the souls of the dead and it's possible we may see something like that happen here. It's also possible that if the Odinson perishes during the Ragnarok, it'll be his own soul for which he attempts to make a deal.
Beta Ray Bill
A fan favorite, Beta Ray Bill is a cybernetically enhanced Korbinite, a race of people who were almost entirely destroyed by Surtur. Bill has bested Thor in combat and was able to life Mjolnir and was given his own enchanted Uru hammer, Stormbreaker, by Odin. It's possible that Bill already made a cameo in James Gunn's Guardians of the Galaxy, and that we may see him join the forces of Asgard in Thor: Ragnarok.