The possibilities are getting interesting. Initially after seeing the trailer for the first time. I immediately assumed that was Stella in some sort of formal dress up. When Luna was mentioned, I figured it was meant to be a red herring over the 'Stella' shot.
Thinking over some of these theories, I found myself believing more that Stella is using Luna as a pseudonym/alias. Referring back to that intro of the E3 2013 trailer it's possible Stella & Noctis knew one another as children, but were apart for so long they couldn't recognize each other as adults (or at least Noctis wouldn't be able to.) We see them meeting each other at the party for peace treaty signing, as if for the first time. Although that contradicts the scene of them as children (assuming they are indeed Noctis and Stella playing.)
Now bearing all this in mind, from the preview we've had of their conversation back in that 2008 clip, she never once introduces her self as 'Stella'. I have a hunch that is the moment she introduces herself instead as 'Luna'. She then invites Noctis to come visit her in Tenebrae. I'm aware that particular clip was most likely from an incomplete script, still I feel it's plausible. Stella seems to play an important role in the events of XV, so there are reasons why she would want to conceal her identity. Possibly for her safety, with tensions increasing, and the state of the nations slipping back into war.
On the flip side, the twin sister theory is equally as interesting. In regards to that one, I'm at a loss, so more story details are needed.
