An ending where the Predator survives purely to satisfy the circle-jerking fanbase would be no better.
While I can understand what you are saying here...I wanted to say that an ending like this could potentially break off into two separate movies. This is what I am saying...
Yes, it could set stage for an AvP 3 - it can easily be done. Fast Forward some odd years into the future - and this time set it in space. Bring back the colonial space marines, predator and aliens and we can have a ball. It can leave the story wide open; you can either continue from AvP 2 or start-over completely with AvP 3.
Now, what if some of the aliens broke Quarantine and left the city way before...well, you know I do not want to post any spoilers since we all pretty much knows what happens but...
what if, IF, a small group of Aliens did indeed break the quarantine, left the city and was long gone before the nuke dropped? If we think along the lines of genetics, there is the potential for more PredAliens with the reproductive system. Therefore, they are loose, create a new hive, begin impregnating humans, and starts a whole new mess - therefore creating a 5th Alien movie...probably Earth Hive.
Again, I'm sitting here at work, bored to be honest and just brainstorming, I don't know =(.