Arnold should be in this. Plain and simple. Anyone who says otherwise you are over-thinking it AND you have a ton of other crappy Pred movies to be your guide. AVP's, Predators.... They were C-level (Predators you can argue was "B"-level, some cool stuff). Pred and Pred 2 are legit movies. You need to bring legitimacy back to this thing. Not some campy comic-book-like irrationality.
Arnold being old is no matter, if anything it brings a sense of REALITY back to the screen. Also Arnold leading a pack of Preds is LOL. No doubt he LOL'd at Rodriguez on that one and said "no thanks".
Shane Black knows the gameplan that got Pred to be legit and he's gonna get back to that gameplan. AKA Arnold. And Arnold knows it too.
It's gonna happen and it's looooooooooooooong over due. Can't wait to add "The Predator" to my movie shelf and finally toss AVP's to the garabage and move "Predators" to the bottom shelf. It's pretty bad when my 10 year old loll's at the movies after Pred 1 and 2, asking after...."Why didn't Arnold come back?"