On the note of zombies, I think it would be a fun idea to see as a mainline thing.
Like one happens to be carrying something and it causes an adverse affect on Humans and wildlife. These creatures are completely alien to life on Earth, and might have something that Humanity just isn't prepared for either they bring it with them or it happens while on Earth. So in the case of Zombies we've seen their blood cause odd mutations in Humans with Hunter Borgia. We've also established now that some Predators don't have the same sense of taboo against genetic manipulations. So something might be developing inside them and or the technology that is facilitating these mutations that might have an adverse affect on Humans. It would also further establish as to why this practice is so out of place among their species (and another reason it's critical they dispose of themselves if brought down). It might very well be that it causes unnecessary stress when creating vaccines and kick start the emergence of disease threatening Predators and prey items.
Kind of like the people coming to the Americas where they end up accidentally killing each other and spreading god knows what because both parties are not equipped for certain things.
Now on film it might be explained that the Predators in their time on Earth have accounted for things like this with vaccines and tech to keep the real nasty stuff held back, but not every Predator follows protocol and we've seen the tech advance since even 87. So maybe the humans find a tomb where that sort of thing wasn't available to them yet and get exposed to something. Or that they capture a mutate and in running their tests the tech works through humans and we get a series of mutations. In the case of the humans they get turned into something awful since it's designed for Predators and not humans. This can be seen as painful since Humans don't have the same pain threshold and the mutations aren't working off the same sort of genes,
In a weird way "zombies" could totally be doable in this franchise.
