I'm not a fan of so easilly killing off staple characters, but if I could pick anyone I'd say Y'shtola has to go. They've "killed" her off once only to immediately bring her back, and then "killed" her again by giving her a disability that is barely ever utilized in how she is written an acted (animation-wise, not voice-wise) that is supposedly draining her life force despite her not showing any strain when she uses it (which she is constantly using it despite being able to turn it off and KNOWING it drains her life force), and then they "killed" her again by having her do the SAME copout she did before only with even FEWER consequences because Deus Ex Grandpa literally snapped his fingers and made it all better again. (Or they could just write her better. Psst, devs, just plagiarize the heck out of Toph Beifong, that's how you write a blind, sassy, magical juggernaut)
Alphinaud too. He doesn't have to die he just need to be written out of the story. Or at least not be on the front lines doing commandy stuff. To this day I have no idea why any of these adults would be taking orders from an impulsive, idealistic child who has REPEATED VERY LARGE FAILURES UNDER HIS BELT, and allowing him to continue to do a bunch of war things. Let him hang back and be a field medic, that would be great and make his character more interesting. Him being the (very pompus/smug/baby) face of the party just shatters my suspension of disbelief.