I like the theory that Yotsuyu's amnesia probably isn't gonna be "cured" with friendship or love, but through sheer hatred. That's a nice twist
But onto more serious matters:
Well the glove of the misterious person seem to match Asahi's gear (saw the stinger again to check). Could always be a fake out, sure, but i don't think it would make for that good of a plot twist. More than one dude in the Garlean Army using the same oufit? AUDIBLE GASP! Our good ambasador with a suspicious surname being more than he seems could definetely make for an interesting plot however.
Now onto the juicy part. I think somebody on this thread mentioned that the smoke that comes out of Byakko when he transitions from tiger to man-tiger bears some resemblance to the visual effects associated with Ame-no-Habakiri. While that by itself doesn't say that much, the lore fluff released about the trial mentions a "peerless samurai" named Tezen, which apparently is a reference to FFXI, where, as far as i could understand based on secondary sources, he was a samurai that housed the spirit of Phoenix on his katana. Seems like a great candidate to be the original wielder of the sword.
What if the Ame-no-Habakiri is deeply related to the Four Lords? Maybe it's used to contain them, maybe it was made by them and gifted to Tezen and for some reason Zenos using it awakens them. The reason i'm bringing this up is... the sword seems to be a perfectly exploitable plot point/mcguffin yet seemed to be discarted so fast. I'm sure it'll make a comeback and this seems to be a nice way to integrate it.