Sorry guys, but for those who believe that their is a new season, quit dreaming. You have to understand that they are only actors, they aren't really witches, so, they need time for other things in their acting career. Not only that Alyssa Millano (Phoebe), and Rose McGowen (Paige) don't want to continue the show, they wanted it to end and now it has they are moving on to future projects. Holly Marie Combs(Piper) is the only one who wouldn't mind making a new season, but the other actors do not have the time.
And to the answer of why Shannen Doherty(Prue) didn't return. It has been proven that the producer said he'll call her for her to get at least one more scene on the show, but he never called which is why we never saw her again.