Duch,Harrigan and Garber could be back but we dont know anything about the plot yet.
Garber became the head of the OWLF and he may need the old guys to help him out with a plan which Harrigan dont like and hates Garber so he's leaving.(Later rejoins because he afraids preds visiting him because he killed one ,so he better prepare)
They have time to prepare because they know the hunters visiting Earth in every ten years and they alredy had Predstuff from P2.
So,if they have Arnold than this is present day in 2017.I prefer for a predator movie to be on Earth in present time.
Will be interesting if they mentions Predators or not.
+They need Alan Silverstri back as well for the score.