I'd love to see Arnold.
But like others have pointed out, he's way too old to fight a Predator. I doubt the Predators would even notice him on a hunt as he once had the strength to take on one (barely), now that strength is gone.
However, Arnie's biggest strength against the Predators wouldn't be physical, but experience.
He'd be a great advisor against them. Though I must ask, Arnie's biggest pointer will be "they see in heat", would Arnie know that other Predators have different vision modes? That could be an interesting point.
"They see een heet, you mast covah yourself in mahd so they don't see youu... theoretically..."
"But, sir, our Spec Ops team did just that and got slaughtered..."
"... geev me a meenut, I need to theenk ahp a nyu plan. I'll be back..."
Ok, I really want Arnie to be in this now.