If possible, how would you Rewrite Kael'thas' plot?
I really think Kael was wasted not just because of his own character, but he got connection with many major figures, it could create a lot of interesting stories among the Horde/Alliance and Nightelves/blood elves.
As much as I hate how he was done in BC, I think the best way wasn't to totally toss away that plot. It could be used for good development with some changes. Like he was under some mind control in BC but he still tried to play double agent and caused some harm to the legion. When he was revived back, he still deeply regret for his doing and tried to make it up secretly. So he spent time and effort to help his people secretly(much like a Happy Prince plot). It could also give other blood elf characters good plot about realize the truth-feel sorry for their prince-change their view. At the same time characters like Sylvanas and Tyrande could also get things to do.