Fan theories speculate what The Last of Us: Part II is about

During PlayStation Experience 2016 Naughty Dog and producer/director Neil Druckmann announced the upcoming sequel to one of the very best games ever created, The Last of Us, via a sublime teaser trailer showing Ellie in a more mature state, playing the guitar after slaying a couple of drifters in an abandoned house.

Druckmann took to the stage later the same day to address some questions, and he confirmed that the game is centered about hate and revenge and that we will be playing mainly as Ellie this time around. He did not confirm or comment on who Ellie is targeting with her hatred or revenge though, but some very interesting hints have found their way online this afternoon.

There is speculation that the game concerns the Fireflies and how they killed Joel's brother Tommy after Joel rescued her in the last chapter of the first game. Learning to play the guitar is one of those things she would have learned at the dam, were Tommy lived, and the window with white drapes and ivy growing through it is a reference to the menu screen of Last of Us, which featured the sound of running water (the dam) and Ellie's retired knife on it after you beat the game.

What do you think of this theory?