I haven't been able to find a translation of their conversation, but it could be a younger Diantha (which would correspond with their gameverse conversation in Lumiose). Or—and this is pure speculation—I think it also bears a certain resemblance to someone else: a younger Sird, though her eyes aren't as narrow.
Sird has displayed a tendency to be involved with just about every villainous team (excepting Team Plasma…as far as we know, since the B2W2 Arc hasn't come out yet), so it's possible that she may have ties to Team Flare, especially since we don't know what happened to her after the DP Arc.
Regardless of whether the face is Diantha's or Sird's or someone else's entirely, what do you guys think about a connection between Sird and Team Flare? She already has the immortality thing going like Xerneas does, what with the armor she gave Archie (and what on earth are we going to have for the Omega Ruby/Alpha Sapphire arc??) and the hints that she might have immortality from Jirachi from ages past. And she has the destruction thing going like Yveltal, since she was a part of Team Galactic and already tried to destroy the world.
So, are we going to see Sird this arc? Or in the B2W2 or ORAS arcs? Or will we have to wait until a Z arc?