agree with your comment on Arnie (it does not make sense to me that the machines would built robots that looks like 60 year olds)

But honestly for a split second I thought Matt Smith would playing the terminator and thought that would be an interesting take on it (The machine building a robot that looks so none-threatening).

As for him being an Action star, I can see him beside John telling him to run, as they run away from a terminator :) . But seriously without more info it is hard to say what he does in the movie. In the original most of the plot is in present day so any future part might be minimal (them sending Kyle back to save Sarah).