I'm sorry but Arnold is the Terminator. The other machines are fine for villains, but not as new main characters. They need to let this franchise die, or at least sleep for twenty years until a new generation with no memory of the original film is going to the movies.
I've heard that this movie will be primarily about the beginning of the war against the machines (before the T-100s became active). If Arnold were to appear, he might show up at the end. Just a thought.