Marvel, in all honesty, is somewhat stupid about casting when it comes to their own studio. They approached Scarlett Johanssen for Iron Man 2 but just for an unspecified female role, they wanted to work with her but tailored a role around her which is an awful idea. They floated Moonstone and the Scarlet Witch by her but ultimately decided on Black Widow. She does a sufficient job as Black Widow but not necessarily amazing in any way the way RDJr is as Iron Man.
I imagine it's something just as boulder-headed when it comes to Vin Diesel. "Hey Vin, the 18-45 male age bracket likes you so we do too. What role do you want to play?" - so silly.
Is that how they went about it for Scarlett...? That's not how you cast roles. Story first, then find out who would best portray the characters (irrespective of how famous they are), then cast them. Nina Gold (casting director of Game of Thrones) should cast everything – she gets it right.
I agree Scarlett is sufficient, but not as great as Black Widow could be. Avengers gave her a lot more room to work with Natasha, but still lacking something.
In any case, I wouldn't be surprised if this were casting for Guardians of the Galaxy, because all of the casting in that has been kinda wonky. Strange choices all around. Zoe Saladena as Gamora seems to be the most appropriate casting in the entire film so far. Hopefully this is for Vin as Groot or Thanos, which I think would be decent choices. But who knows, at this rate they may cast him as Moondragon.