The Beginner Box is great and I'd love to see an Expert Box that allowed for 6th to 10th level play. One of the things Paizo has stated is that they don't want to cannibalize sales of their main RPG.
I was wondering, however, if it would be possible for them to tweak the AP slightly (and yes, I know they are loathe to do that, but bear with me) to allow for the first three installments to work with the Beginner Box and a hypothetical Expert Box.
The first tweak would be to support the Beginner / Expert Box mechanically where needed for the first three books of an AP. I doubt there is much that would need to be added, right? After all, it isn't until after 10th level that lots of the complexity creeps into the game.
The second tweak would be to have the third book (which usually ends around 10th level) of an AP come to some sort of satisfying resolution, though obviously not THE resolution of the story.
This would give people that like the Beginner / Expert Box and want to keep their game a bit simpler use out of the APs -- and maybe even hook them into other supplements. Also, there's a good chance that players will want to continue with the second half of the AP. Provided the conversion is painless enough Paizo would then find people converting to their main product.