There are many cosmic characters in the Marvel Cinematic Universe (MCU). Many that would fit into Thor 3. Here I will be giving ideas for new characters in the movie.
Beta Ray Bill
This is the one character I want to see most in this movie. And he has to do with the Ragnarok storyline, so this movie could include his origin. If this movie includes Asgard being attacked by Surtur, then Beta Ray Bill would be a great addition to fight alongside Thor.
Enchantress is a great character. And Thor need's more powerful characters in his movies besides Lady Sif. Enchantress could work with Loki, or she could work with Surtur. Whatever one, she works in this movie.
Balder the Brave
To give Thor another Asgardian to fight alongside, Balder would be the perfect fit. If Marvel wanted to, they could make Thor and Balder have a past. Maybe the two were good friends when they were young, and would play like they were in a battle against the Frost Giants. Balder is a very different character that I would like to see in this movie.
Valkyrie would be a perfect fit. She is an astounding warrior and a great and interesting character. She would be another great warrior to fight against Surtur and his army.
Lorelei has already been introduced on Agents of SHIELD. Lady Sif brought her back to Asgard as a prisoner. So what if in this movie, Loki and Enchantress break out Lorelei. She can control people's mind and that would be a very interesting dynamic in this movie.
Do any of these characters sound good? Who do you want to see in Thor: Ragnarok? Please, let me know!