If they would not change that the diablo allways drop legendary first time you kill him in quest progress, you could prepare your quest chain just before diablo on lvl 60 now and when RoS comes out exp to level 70 and when you hit 70 kill kill diablo.
Iam planing on getting experience on CotA if it will go fast there, but you shoud be able to get into act 5 aswell if you join friend who is just starting which will add current quest to your story progress even if you did not kill diablo yet.