I think every franchise, trilogy, soap opera, etc, that is divided in several parts to form a whole, needs to be coherent and the action evolving naturally... That said, there are at least 3 aspects (feel free to add more) that will make it or break it for me at the beginning of AVP2:
1 - How does the Predalien (if it is onscreen) manage to grow up, kill and enable the spaceship to falll onto Earth when we already saw in AVP the ship was already out of range (gravity-wise)?
2 - How can there be any survivors of the crash (aliens, predators, facehuggers...) knowing full well that any crash of these proportions will provoke such destructive energy that woould vaporize anything close by (like Alien Resurrection's final demise of the Auriga)?
3 - How can Aliens and Predators fight with surviving witnesses and somehow these events be completely desinformed, covered-up, and plausibly kept a secret by the officials, especially in this day and age of news media outlets and satellites all over the place...?
How can this movie prevent a continuity disaster just waiting to happen?