http://theinstance.net/2013/06/inter...micky-neilson/ around 14:30.
"I'm a big Rexxar fan, I was really really pushing for Rexxar to be in this book. We went back and looked at the history with Chen, and rexxar, and Vol'jin, and all the things he's been through, and I got out voted on Rexxar"
So the author wanted to put Rexxar in the book but Metzen and other writes decided against it. Not only is Rexxar not being mentioned, Blizzard is pretending he doesn't exist, and this expansion would have been the ideal time.
To me this confirms he will not be Warchief.