So...there's going to be a lot of hypothesizing and theorizing here, so be well aware that everything I say could be way off and I don't have a ton of "evidence" to support this. That out of the way, let's do this.
First off: this Link might be a girl. Maybe. He (or she) is certainly the most feminine Link we've seen yet...assuming it is Link. However, several things point to this not even being Link at all...but we'll get to that in a moment. First, we know that Link and Zelda are incarnations of the spirit of the original Hero and the goddess Hylia respectively, and that they are reincarnated in tandem with one another (along with the demon Demise) throughout the course of time. Keeping that in mind, this is either Link or it's Zelda—some people say maybe Zelda is the hero reincarnated this time, but I don't think so. It doesn't fit the lore very well.
That said...that doesn't mean this hero has to be Link at all. At first I was very hesitant to accept that it wasn't Link. I mean, it has his earrings, his pajama colours, his hair, etc...but the more I looked, the less sure I was.
To start, Link is left-handed. This is one of the weaker bits of evidence I have, because Link has been right-handed in two games (Twilight Princess and Skyward Sword), but that was because both used the Wiimote as a basic function in the game. He was even left-handed in the Gamecube version of Twilight Princess. I don't think this game will have the Wiimote as a large focus because Nintendo is going to want to use the Gamepad as this game's main focus, showing off what it can do. That aside, the cross-belt is also over his right arm here rather than his left. So that's (possibly) strike one against this being Link.
A few more things that were off...I noticed that the horse in the trailer is not Epona. Its fur and mane are the wrong colour. Secondly, just...just look at him/her. That is the most feminine Link I have ever seen.
The most important bit of evidence, though: Eiji Aonuma (the producer/director of most Zelda games A Link to the Past onwards) has stated that they never said it was Link in the trailer. That's pretty damning on its own.
That doesn't mean it's Zelda, however. I mean, look at the character! That general design just screams Link, and, personally, I'd be sort of offended if Zelda just happened to look like Link this time around. And that still doesn't explain anything, because the hero and Hylia's incarnation are supposed to exist simultaneously. As a massive Zelda fan, I wasn't happy about this. Unless...this is where my theories come in
Ocarina of Time. The battle that split the Zelda timeline into three. Timeline one: the hero loses to Ganon and is killed. This leads to A Link to the Past, the Oracle games, and the original The Legend of Zelda.
The next two timelines assume the hero is victorious, but it gets a little more confusing. Link defeated Ganon as an adult, but, after defeating him, was sent back to the time where he was a child, continuing from there. In the child timeline, he ventures off to Termina for Majora's Mask, and much later Twilight Princess happens. In the other timeline, since Link defeated Ganon but no longer exists there (as he was sent back to his childhood), it's now a world without Link. This leads to Hyrule being flooded later as there is no hero when Ganon is resurrected, allowing Wind Waker and its sequels to ensue.
Now it gets interesting. What if this game is set in timeline two or three, after Ganon's defeat? In both timelines, Link is no longer around. Now, I have my own preferences for it being set in each timeline, so I'll break this down into both and explain why Link being gone is so important.
Timeline two: This one I prefer story-wise and just in terms of making sense in one specific way. That one way is Zelda. In this timeline, Link has left for Termina, so Zelda is still young and Hyrule is still around. Now, imagine later on Hyrule needs a hero, but none appears. Link is gone. So Zelda steps up to take the mantle. She's older now, she cuts her hair short and pulls it back in a ponytail, dons an earring like Link and takes on his general visage. Zelda is right-handed, which suits the character, and—in a small but important bit...
...—she takes Ocarina of Time Ganondorf's gauntlets. Look at the image above of the new "Link" and "his" gauntlets, then look at Ganondorf's. There is no way that's simply a coincidence. Obviously they're not exactly alike, but that could be chalked up to the creative differences between the games. Even if this isn't true, this at least hints that the game may have much more to do with the Gerudo, taking their designs for the main character. This character's hoodedness is reminiscent of Zelda's in Twilight Princess, and the arrow fired could arguably be some form of light arrow, which is her signature weapon.
Now, timeline three: This is more or less the same, but since Zelda's already older in that timeline it's much less likely. However, we know from Wind Waker that eventually Ganon does reappear and there isn't a hero to stop him, so that leaves open the possibility that a future Zelda took up the mantle to fight off evil in general. I like this timeline less story-wise, but I do like it because of one reason, which is that the vibrant, lush Hyrule(?) we've seen is much more similar to the brief glimpses of Hyrule we get in Wind Waker. A gorgeous, colourful, vast world. It's just the issue of there being no hero between Ocarina of Time and Wind Waker that prevents me from thinking it's this.
Of course, I could be way off the mark here. Maybe it takes place a generation or two later in timeline two, maybe it shows the founding of Hyrule post-Skyward Sword (which would be interesting, but doesn't explain female Link or Zelda as Link), there are tons of possibilities. But, frankly, the idea that the hero hasn't appeared and that Zelda is forced to take up his mantle and tries to appear as him is the one theory I'm most confident in and that I'd be happiest with.
Of course, there's always the possibility that we'll be able to pick the hero's gender, but that would be boring and upset us Zelda-lore fanatics!
If you notice any flaws, please do mention them. I'd love to refine my theories further. :]