Lol if Arnold wasn't even a challenge, who would be a challenge? Whenever Arnold had the drop on the creature, it was never due to his physical strength, it was due to ingenuity. And seriously how much hand to hand combat could there be? We saw a fair amount of that in Predators at the end. Royce punching a predator several times, it's like ok, we get it.
While I would've vastly preferred Affleck or Cooper, I think Franco would be a fine choice. He's better than most of the actors in the Predator series. I'm holding out hope that Ryan Gosling will star. He'd be amazing. Yeah he's filming Blade Runner 2, but push filming back to October or November and you'd be good.