It'd be pretty nuts for him to play a terminator again; he's 63. In very good shape, but still he's not Mr Universe any more.
He could be the human the Arnie T-800s were based on. They could CGI his (young) face onto a body double for actual T-800s as they did in T4 (below).

Wouldn't be for many scenes; they ALWAYS have to have him taking a lot of damage to expose his robotic parts, so then on it's all CGI.
And I don't know anything about "Fast Five director Justin Lin", but the Fast 5 movies are just dumb action pieces, so it's not looking good for a thoughtful extension of the saga. Just lots of shit blowing up. In 3D no doubt.