Vin Diesel, for obvious reasons. He will bring his fan base to the movie and he is more than capable of owning this role.
Depending on which Inhumans storyline they draw from, the 1988 graphic novel depicts Medusa as the wife of Black Bolt and the pregnant Mother of their child. The council is worried about what their child will be capable of and they are afraid that the Black Bolt’s powerful voice may be passed onto his son. The council orders that the child be killed. Medusa cannot let this happen and she abandons her husband Black Bolt and flees to Earth.
Emilia Clarke does an excellent job in Game of Thrones. Her role as Khaleesi and her portrayal as a mother to her dragons is one of the reasons that I feel she would be a good fit for Medusa. The relationship between Khaleesi and Drogo is another reason that I feel she would be a good fit and could make a strong relationship between Medusa and Black Bolt believable.
Juno Temple is an incredible actress and I feel that she could bring a lot to this role.
Anyone that watches Game of Thrones know that you do not want to be captured and tortured by Ramsay Snow. Iwan Rheon plays Snow perfectly on the show and I feel like he would be an amazing Maximus (who is The Black Bolts crazy brother). He would be a good choice because he can play crazy and still be smart. He also has a different style and personality from Tom Hiddleston, this could help to avoid comparisons to Loki.
Luke Goss was amazing in Hell Boy, and Karnak isn’t just smart, he’s also a martial artist. Luke Goss can definitely look the part and I think he could pull off the action too. This was also probably the hardest part to try and cast, I’m not sure how Marvel will approach this character.
He would have made a good Aquaman, so maybe he could make a good Triton.
I think Joe Manganiello deserves a shot at a CBM character. It would be interesting to see what he could bring to the table as Gorgon.