Funny you mention that - I'd have cast Christian Bale as Walton Simons, myself.
^ Going slightly off-topic. Now that you've mentioned it I agree that Bale has the voice and look to make a very good Walton Simons. I think Reeves can pull off JC just by being a slightly different Neo. In the game JC was pretty much monotone, emotionless and always wore shades.
My really expensive cast (Deus Ex will probably take up 3 movies) would be.
Main Cast
JC Denton - Keanu Reeves (1-3)
Bob Page - Kevin Spacey (1-3)
Walton Simons - Christian Bale (1-3)
Tracer Tong - Tony Leung (2-3)
Maggie Chow - Li Bing Bing (2)
Morgan Everett - Morgan Freeman (2-3)
Paul Denton - Eric Bana (1)
Alex Jacobson - Anton Yelchin (1-3)
Gunther Hermann - Arnold Schwarzeneggar (1-2)
Anna Navarre - Milla Jovovich (1)
Gary Savage - Mitch Pileggi (3)
Support cast
Jock - Jeff Fahey (1-3)
Jaime Reyes - Michael Peña (1-3)
Joseph Manderley - Alec Baldwin (1)
Sam Carter - Sam Shephard (1-3)
Juan Lebedev - Vincent Cassel (1)
Nicolette DuClare - Eva Green (2)
Chad Dumier - Djimon Hounsou (2)
Max Chen/Gordon Quick - Chow Yun-Fat/Andy Lau (2)
Janice Reed - Mary Lynn Rajskub (1)
Stanton Dowd - Ian Mckellen (2)
Sandra Renton - Christina Ricci (1,3)
Tiffany Savage - Maggie Grace (3)
Cameos
MIB - Will Smith and Tommy lee Jones