As you may or may not know Kul Tiras was originally planned to be in Cataclysm but some time during the beta they scratched the idea, if you look at old beta footage though you can still see Kul Tiras troops as well as a portal to the location. Considering that it was planned in Cataclysm Kul Tiras seems to be a location that Blizzard has wanted to implement into the game for quite some time. The question though is when if ever will we get to see it?
As for whether or not we'll see it in Legion, I hope we will but I also doubt we will. Blizzard has announced some of the main characters that we will follow which are Illidan, Maiev, Genn Greymane, Sylvanas, Gul'dan, Kadgar, Anduin, Xavius, Athissa and Dargrul. None of these really have any ties to Kul Tiras not to mention a lot of other stories also seems to be going on, Ysera, Cenarius, Malfurion and not to mention there's a new faerie dragon model which suports my theory that Brightwing will make her first appearance in Legion. Not to mention Lunara, Odyn, Black Rook Hold, the blue dragonflight, Neltharion, a new elven race and who knows what else. To put it shortly I do not think we'll be seeing Kul Tiras in this expansion.