For some reason I don't think Maria will officially retire yet but will rather use her active player status for some other off court benefits. Serena and Venus will not retire before Serena starts losing very early at slams so it becomes obvious that she is done winning titles. She plays like 5-6 events a year so it's not like tour life takes much of her time anyways.
I expect Cibukova to go, as well as Jankovic's officiall retirement, perhaps even Azarenka and Wozniacki. Would not be surprised if we get one sudden retirement such as Stephens/Muguruza.
Ana Ivanović Schweinsteiger
Roland Garros 2008 Champion
Roland Garros 2007 and Australian Open 2008 Finalist
World No.1 in 2008