Fox has already done the crossover and it did not work out. AvP could have been good but it was just ok (still I prefer Bishop Weyland to Peter Weyland), and as for AvP: Requiem, it was a farce.
I pray to the Engineers that Fox does not take the box-office success of MCU (and to a lesser extent, DEU) as justification to bastardize Alien and Predator again.
We don't need to see them both in the same movie because we do not have the attention deficit of a 9-year-old, constantly needing to be kept interested with umpteen primary characters.
PS: no offence to 9-year-olds; some of my best friends are 9-year-olds. So there you go, I'm not ageist.