this is my speculation
But I believe Illidan isnt dead.
after watching to Cinematic, Illidan seem to be able to talk after his defeat, and hold himself up. This seem to suggest that he survived the fight, but was unconscious like during Arthas fight.
Also with the Highborne being brought back into the night Elf community,
it seems as if they would be more open to Magic users like Illidan,
and I believe he will be the new Demon Hunter trainer in a future X-pac against the Legion.
edit:
Cool Story?