While the idea is neat, given what we've seen so far, I don't think it could happen without feeling like a complete asspull. Like on one hand, we got all this hopeful buildup between "There's always a way," Mickey saving her from fading, and that Sora imagery at the end of the 0.2 intro... and then it ends with Aqua basically getting defeated by the darkness? Sure, no one expects the diabolus ex machina, but I can't see it sitting well with anyone. Besides maybe the darkness fanboys.
Yeah, that suggestion just reads like overwhelming edge for the sake of it rather than being interesting or actually tying into the narrative.
Also personally don't like the term mary-sue being thrown around cause it's just lazy.