These are my picks:
First is Sam Worthington for Master Chief:
Next is Keith David(who voiced him in the game)for Arbiter:
Tricia Helfer for Cortana:
Emily Blunt for Miranda Keyes:
Denzel Washington for Sergeant Major Johnson:
Clancy Brown for Gravemind:
Terrence Stamp(who voiced him in the game)for The Prophet of Truth:
Tim Dadabo(who voiced him in the game)for 343 Guilty Spark:
Geoffrey Pierson for Captain Keyes:
and Ron Perlman(who voiced him in the game)for Lord Hood:
Well that is my fan cast for the Halo movie.
Ozy out.