Like Aaz said it has been planned for a long time with Robert Rodriguez attached to the project. I know before AvP 2 came out he had written a script for an AvP sequel but it was rejected. The reason being that it was too ambitious, like an epic space battle between the two alien races in the future. I can't give a source because i read it so long ago, but I think even Robert mentioned it in an interview. He also has said on the extra features on some of his dvd's that the original Predator was one his most influential movies and made him want to be a director. Looks like he is pursuing his love for predator movies.
I would rather have an AvP 3 with Rodriguez directing than a reboot. The crossover franchise already sucks so maybe a semi talented director like Rodriguez could bring some coolness to it. A reboot or remake is just a bad idea. The original is perfect and it won't continue the saga. But being a fan of Rodriguez, I'd like to give him the benefit of the doubt, and I know i'm the only one on this. But only if he directs it, which like the article stated, is unlikely because he has so many other projects to juggle right now.