Something seems to be happening behind the scenes at Naughty Dog. Two days ago The Last of Us Game Director teased seeing an "awesome" prototype, and today the concept artist that created many of the game's artwork shown something that looks a lot like a grown up Ellie.
Update: Marek Okon just posted an update on facebook:
Lol. This is how you know you are teasing your stuff the right way
So before its gonna get blown out of proportions - its gonna be strictly personal image.
Is it about TLOU2? Not really... Maybe... Its classified ;P
But seriously its gonna be just a fan art ;]
He still seems to be teasing something, but we can't know for sure if he actually knows something about a possible sequel (he worked on the game, after all) or not. But there you have it, straight from the lion's mouth. At this stage your guess about it is as good as mine.