Not really...at least with Illidan returning at least it has a valid reason compared to kael'thas' "it was just a setback!" garbage, illidan is half demon (looks like part nathzrim to me) and they can be brought back from the dead since their soul wanders around in the nether when the body is destroyed (mal'ganis says hi)
Kael'thas never died in The Eye. After "heroes" killed him, Priestess Delrissa healed the elf, using some demonic-technology (gems, fel energy, etc...) and he moved to help his new master.
Why he isn't a raid boss in SWP? I don't know, He should have new demonic powers too, like Illidan, he could be a mix between a demon and mage, and could be really awesome to see the other face of Kael'thas (another face would me something new that we didn't know, maybe a new face to face between him and Jaina would be funny).
Problem about that, there aren't any quests related to that, only some hints here, some new text in the webside, and some writtings in fan-webs. Really hard to understand if you are only playing the game.