I'm not saying I think Lucina is
definitely an alt. She could very well be a clone character. However:
1. Sakurai is unpredictable, yes, but every single newcomer so far has been very unique. Why now add a clone character? I understand that Lucina, in Awakening, is attempting to mimic Marth, but still, every newcomer so far has been extremely unique (including Robin), and most are given some sort of gimmick.
2. Lucina is
so similar to Marth that even her idle animations are the same. At least Toon Link is given different animations.
3. The "?!" at the end of her tagline indicates some sort of uncertainty about her status. It could be because we were expecting Chrom, or it's because she isn't a full-fledged character.
4. We know some alts in this game are much more complex than just model changes, as female Robin is given a new voice entirely. It's very possible, in my opinion, that Lucina, as a Marth alt., could be given a new voice along with new taunts (her mask-removal).
- I'm also aware that we have never had alts. that were entirely different characters. Perhaps that's what this trailer was introducing? I don't particularly like this precedent if it is true, but Lucina is similar enough to Marth in looks and her clone-ness is canonical (she pretends to be Marth in Awakening).
5. Perhaps Sakurai is treating Lucina as a separate character in the trailer and site because introducing alts-that-are-different-characters (like Dark Pit) is being saved for a later date.
I'm also really leaning towards the alt. theory because Lucina is, looks-wise and moves-wise, a female version of Marth. And because Robin was given both a male and female variant, I'm thinking Lucina is being treated as the female variant of Marth.