Apparently Arnold Schwarzenegger has read the script for Shane Black's The Predator and declined a part in the film, because he didn't like the script. In an interview with Yahoo!, he said that:
"They asked me, and I read it, and I didn’t like it, whatever they offered. So I’m not going to do that, no. Except if there’s a chance that they rewrite it, or make it a more significant role. But the way it is now, no, I won’t do that." So it's uncertain whether he meant that the script was bad or that he wasn't happen with the amount of screen time he would receive."
Black responded to a fan on Twitter about it, saying that he hoped it wasn't the actual script. See tweet below.
"They asked me, and I read it, and I didn’t like it, whatever they offered. So I’m not going to do that, no. Except if there’s a chance that they rewrite it, or make it a more significant role. But the way it is now, no, I won’t do that." So it's uncertain whether he meant that the script was bad or that he wasn't happen with the amount of screen time he would receive."
Black responded to a fan on Twitter about it, saying that he hoped it wasn't the actual script. See tweet below.
@iamJermaineP @MPredator24 Me too... I spoke with him, I think it was that he didn't have enough to do. Enough presence in the film.— Shane Black (@BonafideBlack) April 6, 2017
Another report from Alien vs Predator Galaxy is saying that his character Dutch would have shown up at the end of the film to set up a sequel. This would have most likely have led to more screen time in the sequel. There may still be time for him to change his mind while the movie is in production, if his appearance is indeed at the end.