He's not going to join the alliance, he's going to become a neutral hero (like Thrall, Khadgar, and Malfurion) after he redeems himself.
I don't think they will kill off Illidan.Atleast i hope not as he is my favorite hero.I will settle with him helping us defeat the final boss of the next expansion (most likely Kil'jaeden) while forming the faction illidari (which horde/alliance will work on gaining exalted a-la knights of the ebon blade in icecrown) after which he will leave us until we face off Sargeras on Argus.
Just my speculation.