Been a while since I've read any Hellblazer comics and so I've only featured the characters I can remember at the moment.
MAIN CHARACTERS:
John Constantine- Kevin McKidd
I think Kevin McKidd look remarkably similar to Constantine from the comics. I think his age is suitable for the character and if you've seen him in Rome, then you know that he's a very talented actor who is being wasted on Grey's Anatomy. I think he could portray the more cynical, darker take on the character a rebooted film would need.
Chas Chandler- Gerard Butler
Putting the fact that Butler probably wouldn't want to play the sidekick role as Constantine's closest friend, he is also Scottish, looks remarkably similar to the character and well... he isn't Shia LaBeouf. He's 4 years older than McKidd but I think they look the same age and feel they could have a great on-screen rapport.
Papa Midnite- Idris Elba
A great actor who's been producing fantastic performances lately. Djimon Honsou was great in the role but for a reboot I think Idris Elba would be amazing as the crime lord and Voodoo specialist.
VILLIANS:
First of the Fallen- Daniel Day-Lewis
If you've seen any Daniel Day-Lewis movie, you'll know how superb an actor he is and I think he would be terrific as the first being banished to Hell who is after gaining Constantine's soul. Another option is Jason Isaacs.
Nergal- Joseph Gatt
Nergal is a demon who is a recurring character in Constantine. I assume that with prosthetics/make-up and/or CGI, Joseph Gatt could play the role well, especially since he has been cast as a villain in Thor, and so I assume he can act. Otherwise, Doug Jones could be suitable.
MINOR CHARACTERS:
Gary Lester- Mackenzie Crook
Gary Lester is Constantine's oldest friend and fellow former band member of Mucous Membrane. Crook is British, resembles the character, could act as some comedy relief and there is only a small age difference between Crook and McKidd.
Cheryl Masters- Sienna Miller
Cheryl is John's only sibling, being 7 years younger than him, and occassionally appears in the comics. I think she would be an interesting addition to the film. It's a minor role and at the moment I can't really think of anybody else besides Sienna Miller (because of her look, not really because of her acting skills) who could play the role, so if anybody has any usggestions they would be welcome.
Gemma Masters- Emma Watson
Gemma is Cheryl's daughter and therefore Constantine's niece. As she grows up, her behaviour becoming increasingly like Constantine's. I think Emma Watson could be decent in this role. She's a capable young actress who could age with the franchise, becoming more of a prominent character in later films.
Swamp Thing- Ron Perlman
John Constantine first appeared in an issue of Swamp Thing, and makes occassional appearances in the Hellblazer comics. I think it would be interesting for the character to make a cameo in this film (with the possibility of a spin-off about the character), and could work as part of John's origins. Ron Perlman has the right height/physique for the character, certainly doesn't mind prosthetics/make-up and could play the character with the right attitude. His voice is also perfect in my opinion.
Ellie (Chantinelle)- Gemma Arterton
A succubus befriended by Constantine. Not a major role again and Gemma Arterton is a decent actress and has a fair resemblance to the character so I can't think of a reason why she wouldn't be good.