Where do marquee names usually go in Marvel Studios films? Villains, and supporting characters.
Vin Diesel will never play a "supporting character." So what remains is far more likely to be a villain role than a hero. Vin plays heroes and antiheroes, but I've never seen him play an outright villain. Maybe this would be a first for him.
I could see a villain being a possibility. He has a distinctive voice but I doubt they would hire Diesel just for his voice, since it would seem to be a waste of a perfectly good action star. That rules out Rocket and Groot, but he could still be Thanos depending on how plan on doing that character. I could also see him as Baron Mordo.