Though it would be great to have Arnold back, but really, I don't see it happening. He has other franchises to think about, examples being True Lies and Terminator. And plus Im not really in favour for having him return, it wouldn't fit. Because, in a way, what would have been the point of # 2? It wouldn't connect to Pred 1 and 3, only that a predator was in LA sporting.
I say keep the trend, of having different actors(or actresses) to fill the role of the hero. Having Arnold return would be too nostalgic, for my tastes anyway.
But then again, Arnold could bring the Predator franchise out of that pit that #2 dug for the series(unfair, buts thats hollywood).
Guess what Im trying to say is that I wouldn't complain if Arnold came back, but that is not the only option. There could, afterall, be a Alien vs Pred movie hitting the screens first.(maybe have Arnold in that? or is that fanancially impossible?)