2. Groot
I think this is an unlikely choice, however it is certainly possible. The only characters left to cast in next year's Guardians of the Galaxy are members Groot and Rocket Raccoon, with David Tennant in talks to voice the latter. Groot could work if voiced by Vin Diesel, purely because of his gruff voice in general, which could work for a character like Groot. The main reason that I think this may not be the character Diesel is playing is that he said it was really big news, and Groot isn't a really well known character at all. Maybe he was referring to how big the character is in terms of size, but I think it's unlikely we will see Vin Diesel bring life to this character, though it is certainly not impossible.