There did used to be a line just for APs, but it was a fiasco. Guests could not understand that AP only really meant AP only, not AP + all friends and family who aren't APs but who are with the AP and who want a shorter/faster moving line. So now, the lines are for everyone, regardless of entry media.
I don't see how a third gate exclusively for APs would make a significant difference in lowering the crowds. If the last two weeks of 2016 were any indication, all APs except Signature, Signature Plus, and Premier were blocked, but the parks were both very, very crowded. During the summers when the lower passes are blocked, the parks are still very crowded.