IGN have got an eagle-eyed journalist in their crew, a man able to see something that didn’t match with The Last of Us. During a video playback (The Last of Us Dev diary), this reporter peeked something strange in the background. The screen show two characters wearing clothes for the 18th/19th century. Take a glance:
This game could be related with The Order: 1886. Sony had registered this new brand weeks ago. So, this game could be the new PS4 project for the studio or ND could be helping Guerrilla with The Order: 1886. According the rumors, this new IP would be developed by Guerrilla Games.
I would like to add something; Sony presented a tech PS4 demo with a different Naughty Dog game two months ago. Maybe was only a tech demo, not a future real game.