As you know Arnold Schwarzennegar was the star of Predator, but decided not to come back for another installment. Do you remember who took his place for the sequel? Danny Glover. Yes, I said Danny Glover. I thought I was dreaming or might have made that up when I recently remembered this, but I checked and it's true.
I know movie sequels can be a risky thing. Some series like Star Wars, LOTR, Harry Potter, and Spider Man do great at the box office, but many are just horrible and end up tanking. Speed 2 comes to mind, which has to be one of the worst sequels of all time. I don't think it was because Keanu Reeves was replaced by Jason Patric either. The Superman and Rocky sequels each got worse after the 2nd or 3rd installments too.
I feel kind of bad when an entire cast comes together except for one actor who holds out. It may not be an intentional thing or because an actor thinks they are a prima donna, but it is still noticable when you see a main character missing. I recently saw that the 4th installment of Pirates of the Carribean is missing Keira Knightley. Maybe she just has enough sense to know when to say when.