I am absolutely IN LOVE with the newly revealed KH3 theme song!!!!!
I agree with you that this current version is hauntingly beautiful and it would be neat to hear it in this current version as the opening song too, but if history proves right then, as with the previous games, the opening scene for KH3 will most likely feature a remixed version of this song.
P.S. Going off-topic for a moment, did you notice that Riku's new outfit and slightly shorter haircut in the new KH3 music trailer makes him vaguely look like Noctis from FFXV? I saw some people on youtube commenting on that and once I compared both Riku's new look and Noctis I could totally see it!