I prefer to have them thrown into the lifestream during the earthquake against their will and Aeris would be forced to reveal the knowledge she gleaned from the Forgotten City
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I like your idea a lot, it would make Aerith's character feel more real, after all, the prospect of spending the rest of your life disconnected from reality, with the voices of thousands of dead souls screaming at you, all at the same time, constantly bombarding your mind with a incompressible mismatch of their memories, regrets and fears, its probably one of the most terrifying things anyone could imagine, it will be quite realistic for Aerith, to be way too scared of the possibility of failing, to even tell Tifa the true about Cloud's condition, but at the same time feel a desperate need to help her friends, in the mist of that moral dilemma, watching Tifa and Cloud fall into the lifestream during the earthquake in Mideel, knowing that they will be forever lost if she does not act immediately, would be the "push" she need to overcome her fear and face the danger of the lifestream.
One issue with your suggestion though is (not sure whether you are aware or not) that this directly clashes with the optional conversation between Aeris and Cloud at the Chocobo Ranch in Disc 3. There, she says that she doesn’t want to be a Cetra because it entails putting the interests of the planet above all else and that she just wants to lead a normal life. She even asks Cloud whether she is being selfish for harboring such thoughts. That conversation ought to be removed if your suggestions are actually taken on board.
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I was not aware of this optional conversation, thanks for telling me about it, I like it a lot and think it is perfectly in character for Aerith, but I don't think it clashes with my suggestions, in fact, I think it rounds up Aerith's character arc even more, the way I see it, her arc is not about accepting her fate as a Cetra at the cost of abandoning her "human" life, her arc is that she was always a Cetra and she must learn to accept that part of herself, (according to Elmyra's flashback she used to vehemently deny having any powers when she was a child) but that does not mean she has to forsake all the other parts of her personality, she could achieve a balance in which she uses her heritage for the good of the planet, but still lives her own life in search for her own happiness, and to do that, she must rely on the support of her friends, specially Cloud and Tifa.
Achieving a healthy balance between fulfilling your responsibilities and pursuing your passions and dreams is a quite realistic character arc, and a very relatable one in my opinion.