Maybe it is important to note that Lucina says a line inthe japanese trailer: this is the hero of light's power. Before attacking C Falcon.
Lucina has the Falchion and is dressed as said hero of light. If viewed from this lens it seems to indicate she is a costume of Marth, but needs an image on the website since she is a character, not just a different version of marth in the games of origin. Notable circumstantial evidence: Moves have the exact same animations as opposed to her unique ones from her own game, including her idle stance. Exact same height as Marth. Exact same moves down to the details when compared to Marth.
The Falchion was reforged in Awakening, which means it's slashing animation will indeed look different if it's Lucina when compared to her base Marth. This is not proof that she is separate from Marth at all.
Add in the simple fact that Sakurai is trying to avoid clones, he would never have Lucina have a copy moveset, especially since he went out of his way to leave Chrom out of the game as playable.
He picked Lucina because he also voiced his desire to add more female characters.
I do not know why the English trailer changed her line, but she references having Marth's power. Combine that with her lack of screen time as a newcomer and the '?!' While having the exact same animations as Marth save a few...
Not to mention Sakurai clearly stated many more characters would have alternate costumes, I would say Lucina is a costume which has the announcer change his lines.
She foreshadows Dark Pit joining and maybe even Roy with the options to change special attacks.
My two cents.
Remember her line is: 'this.. Is the hero of light's power! ' and preps her sword after which we get the 'Lucina joins the battle?!'
Subtle hints. Anyone who has followed Sakurai over the years knows this may be him hinting at new game mechanics.
source:
https://www.youtube.com/watch?v=Lf9ZxmcUYWs