I'd love to see the past interactions between the Predators and Engineers, not in a "they created them" way, but maybe show if there was a first ever encounter between a Predator hunting party and a group of Engineer scientists. I'd like it if the franchise explores the idea of coincidence, a thing called "Convergent Evolution", look at the Thylacine and the wolf, both have very similar morphologies despite one being a marsupial and the other a mammal.
I know it's an Earth animal, but the idea of convergent evolution is fascinating, and it'd be interesting to explore the idea that the Predators are this one in a billion chance that happened to form.
I wonder, which race is older? Both are truly ancient and predate the human epoch, even Rage War touches on the idea that a Predator space habitat is older than humanity itself, the audio diaries of AvP2010 also say how the Predator ruins predate modern hominids. And we know that Engineers predate us too.
I have this feeling that Engineers are a bit older. But as far as the galaxy's concerned, these two may be the eldest race, maybe alongside the Dog-Aliens from the new novels.
I just love this idea of us being this relatively new and young species, we're like little toddlers just stepping outside for the first time...