That's interesting about Borg. The implication is that this would be after the events of Predator 2 and that being able to more or less time travel is fairly recent. I wonder if this has any bearing on the wormhole technology seen in The Predator. That the ships we see prior going as fast as they do is what they did before and that the worm-hole is this time-travel technology.
If that's the case then it'd be the easiest explanation for all the crossovers. They hunt Aliens by being able to travel to different timelines and even bring some with them which would explain the existence of the skull and spear despite the Xenomorph being created much later. Worth noting that Lasershot has a Terminator skull on his belt and while I doubt it's implication of a crossover that would be an explanation for the tech as well.