Naughty Dog have already started development work on The Last of Us 2, according to Nolan North, the voice actor behind Nathan Drake in Uncharted 4: A Thief’s End. The Last of Us 2 existence was confirmed by Nolan North during his interview at Metrocon 2015.
When a reported asked Nolan about his other future projects with Naughty Dog (apart from ongoing Uncharted 4: A Thief’s End), he replied:
“For now the last one. I know they’re doing The Last of Us 2, but my character in The Last of Us had an untimely demise”
Nolan North has been working with Naughty Dog for a very long time so its understandable that he is well aware of other unannounced projects that are currently in development at Naughty Dog. So we should definitely take this as a confirmation of The Last of Us 2 existence..
You can check out the Nolan North Interview at Metrocon 2015 in the footage below, at 1:06:30 mark he talks about The Last of Us 2.