I don't really think there's a continuity where the Alien isn't known to the Predator's generally. Maybe they're really rare, and it's not like in the comics where Predators have aliens... But Predator 2 fairly certainly, at least in my mind, negates that chance.
However, having that moment where the Predator doesn't know, and the Alien doesn't know.. and they both have that "Whoa... What? What are you?" moment could be a very dramatic moment. Similar to that moment in Terminator 2 when T-800 and T-1000 first engage each other and they have that "gridlocK" moment.
And yeah, that would be for the one movie that you could take advantage of those things, but... Let's be real if there is a relight for the AVP franchise, where they actually get it right... It's from square one. The Paul Anderson and Strause movies won't be under considering. At least, not by my estimation. I'm not saying it will be a reboot, but it will not consider their parameters on itself. It will be AVP again, not AVP 3. So why not have the Alien and Predator not expecting to intermingle in the first film, and then have the second film up the stakes ala Aliens and Predator 2. Have the big battle between the two species later. Have the first one be about the personal test of mettle, will, skill, and capabilities of one another.
Also I don't know that I agree that the Predator only has a couple of things to really know. I think we don't even really know everything about the Alien. Its danger is in the unpredictable nature of the creature.
On the flipside, I think the Predator as a character is full of wild cards. Again, down to the personality any given writer wants to play with. Young, old? Fast, slow? Considerate, aggressive? Rookie, experienced? All of these parameters can dramatically play with how the battles go.