The Legendary is a dagger.
Link to the article
EDIT NOTE: It would seem that the interview that this was from was taken down. It is possible that his was an old interview which never should have been posted.
J. Allen Brack, Lead Producer of World of Warcraft: There's one more piece which is an announcement that we're releasing a rogue legendary weapon. Specific for rogues, so it will be a dagger. There will be some sort of quest, traditionally we've done these quests that players need to participate in order to create the legendary weapon in this case we'll be focusing on rogues that will try to make that happen.
The Escapist: Do you know what it's called yet?
Brack: We do not?
The Escapist: How does it fit into the lore?
Brack: We haven't decided on the name and how it's going to work, we just know that we really want a legendary weapon to come out of this tier and we really want it to be a rogue dagger. That's really all we know at this point. As time goes on we get a lot more lore, a lot more story, an "in" to the creation of all the legendary weapons. We really did a lot with the legendary axe that you got out of Lich King and then we've had other legendaries with the Cataclysm tiers that have.
First I would like to say there isn't much information on this legendary. When more Information becomes available I will update this thread, but for now its pure speculation on the little information blizzard has given us.
This is Currently the only Information we have about the 4.3 legendary.
Now due to past legendaries and this statement I have narrowed down what legendary it would be. It is a 50% chance that it is a hunter weapon.Q: Will we see a tanking Legendary sometime soon? – Pedoso (NA)
A: The tanking community both loves and hates when this question comes up, but it received a lot of votes, so we’ll answer it. The answer is not soon, but probably eventually. The problem with tanking legendaries, of course, is that the shield-users and non-shield-users tank with different weapons. That’s not a deal breaker, but it is a consideration. We could allow the legendary to be transformed from a one-hander to two-hander or we could just design an item for a more narrow audience (such as a shield). The 4.2 legendary has fairly wide appeal, and the 4.3 legendary will have much more narrow appeal. We don’t want to fall into the trap of making legendaries too formulaic.
List of what it can't possibly be and why.
1. No 2H Strength weapon due to Shadowmourne in wrath.
2. No healing weapon due to Val'nyr in wrath.
3. No tank weapon due to blizzard stating that it is not.
4. No caster dps weapon due to them just getting one
List of what it might be but it completely unlikely.
1. A 1H Strength (weapon this is unlikely due to warriors and death knight just getting shadowmourne).
List of what it most likely will be.
1. A rogue dagger (Unlikely due to them having warglaives, the QQ that would ensue, and it seems a little to narrow)
2. A rogue/enhance shammy axe/mace
3. A feral druid/hunter polearm (won't be a staff due to a staff legendary just came out).
4. A hunter ranged weapon (see rogue dagger but replace warglaives with our legendary bow).
Now 2 and 3 are the most possible in my mind as they would be the best fit for the information we have been given. Both enhance shaman and feral druid have been long due to have a legendary.