I know that you don't like to see single character fan casts, but the Batman Series has been done to death, so I will spare you that. However, I have also chosen several different actors who I think could pull this role off. On to the casting.
I would love to see what Christopher Nolan would and could do with Solomon Grundy. I have seen him show up in a few fan casts and at first I wasn't too big on the idea, as I'd always had the vision of The Riddler showing up due to the fact that he is more "real world" that some of the other villains. But the more I thought about Grundy, I think Nolan could make it work.
I am not pretending to know everything about this character, so if I get something wrong about him, please feel free to correct me. I wouldn't write this character as a brainless brute that we have seen in past animated shows, but more along the line of what Marvel recently did with the Hulk in "Planet Hulk." Someone who is at least able to communicate and not sound like a total tool, but would still be able to do some damage in the strength department. I think Nolan could also make Grundy scarier than hell! I love the second image I posted above. He has that "zombie" look to him, but still capable of some coherent thoughts.
As far as actors who could play the part, there are only a few that have the size and even fewer who could pull off the acting abilities required to stand up against the likes of those already included in the film. Here are some that I think might be able to pull it off.
Tyler Mane is most known for his portrayal of Sabretooth in "X-Men" and more recently as Michael Meyers in Rob Zombie's retelling of the "Halloween" series. Some may not know that Mane started out as a wrestler, Big Sky, back in WCW in the late 90's. He is 6'9". In most of his roles, he rarely has speaking parts, but I think he could work. Mane is also who most people have cast as Grundy. Thanks for the idea LEEE!
Daniel Cudmore, known for playing Colossus in the "X-Men" films and has recently starred as Felix in "Twilight: New Moon." Again, not much in terms of speaking parts, but the dude is huge! Standing at 6'8" and well over 200 pounds, he is rather imposing. As far as his acting abilities go, he is improving and could use work, but with the group of actors that star in the Batman films, he'd have some great mentors.
Glenn "Kane" Jacobs is known mostly for his WWE gimmick, but has starred in the film "See No Evil" and while his acting was not great, it was actually better than I expected. Jacobs stands at 7' or just shy of that and is right around 300 pounds. Needless to say, he fits the size requirements. Not to mention, they could save a lot on make up!
Ralf Moeller, who portrayed Conan in the television series of the same name from the mid 90's, is included as a more "seasoned" choice. Ralf stands at 6'5" and is no stranger to action movies; He has acted in films like "Gladiator," "Pathfinder" and "The Scorpian King."
Here we go with another wrestler, but one that actually has a bit more acting under his belt. Dwayne Johnson, at 6'4" is very capable of handling such a role. His voice is one that could be altered well and still come across as dark. His recent roles have been more of a comedy/family style, but I think he could do it.
Finally, Kevin Durand. Yes, he played The Blob in Wolverine, but I think he'd be great in this role and has proven in roles like Gabriel in "Legion" that he can do the action scenes, and do them well. I didn't realize how tall he actually is, standing at 6'6" and has a good build.
I don't think Grundy should be the only villain, or even the main villain, but I think a prominant role in the next Bat-flick would be cool to see and rather un-expected. What say you?