My Warcraft knowledge is a bit rusty, so forgive any misconceptions I may have about the current story and timeline.
In the new Legion cinematic trailer, we see (alternate) Gul'dan interacting with a large green crystal at the Vault of the Wardens that contains Illidan Stormrage (see: events after his "death"). To my knowledge, Gul'dan has no idea who Illidan is due to the fact that, in the prime timeline, he was dead long before Illidan was set free and in the alternate timeline, Illidan is presumed to have never made it to Outland (and orcs had never made it to Azeroth).
My question is how did Gul'dan find Illidan? Was it by chance, or did he specifically set out to look for him? And if he did set out to look for him, how did he know he was in the Lost Isles?