I remember the official casting call was for the main 5, Coby and Garp. The fact that those two are important enough to cast early makes me think they migh put some focus on Coby's cover story training arc with Garp as well, to show the workings of the Marines.
I can definitely see Jared as Shanks. He's got the right look and age. We know he can be comedic, but also able to nail the "cool-headed yet angry under the surface" that's so important for the character.
I could see someone like Nicolai Coster-Waldau doing well as Shanks too.
I love Jemaine Clement and everything he touches. I think he might not be able to be quite bat-shit crazy enough though, his comedic delivery is always much more low-key and dry.
In terms of voice and just nailing that craziness factor, I thought of Ken Jeong. But that might be way too meme-y of a choice.