Some theory is that alt universe Illidan on Azeroth is going to be used to help defeat the burning legion, either this expansion or the BL expansion.
There's even some hint towards our universe Illidan surviving the BT raid. During the greenfire questline, Illidan's demons still reside in Outland and the warlocks question why since he is dead. It's just speculation but its better than nothing. A lot of people do want Illidan to come back as his story wasn't told very well during TBC compared to Arthas for example.
Arthas' story was in my opinion one of the best told in gaming, not so much for Illidan which is why bringing back a character like Arthas would be a very bad idea whereas, it seems perfect to tell some more story about Illidan. Illidan was never really a true villain though. He did some bad stuff but even as a demon, he helped save the world and he even liberated Outland and the broken still residing there from the legion. He's just a guy who's out to save himself and his followers, not purely evil.
The rest on the list are characters we don't care too much about or are just side-characters.