I remark that

• it has recently been pointed out that Harry's Patronus v2.0 is powered by his life as well as his magic and that this (at least according to Voldemort, so obviously it's true) makes it more powerful than it could have been if powered by just his magic
• even the small fraction of his life he was able to give up on the spur of the moment was enough to restore Hermione's life and magic, which even Voldemort was unable to do on his own
• in canon, central to Harry's ultimate victory is his willingness to die

and suggest that if your prediction is correct, what powers his absurdly powerful spell may be the sacrifice of the whole of his life and magic.

(Hmm. The power of a potion in HPMOR is determined by what went into making its ingredients, a curious and probably important discovery that hasn't been applied yet for anything other than winning playfights. What went into making Harry was, among other things, the power and ingenuity -- and in some sense even the life -- of Lord Voldemort. Maaaaybe.)