The first game was an absolute smash hit, and not just financially, but also on a storytelling and gameplay level. There aren't many post-apocalyptic, quasi-zombie games that can hit you in the feels as strongly as The Last Of Us did (repeatedly).
It's a foregone conclusion a sequel will arrive, and while not officially “revealed” yet, its the worst kept secret in the industry that this game is in development. Somehow part 2 wasn't announced at E3, despite all indications it would be, but I don't think we'll be waiting too much longer.
For those who can't handle the anticipation, Days Gone looks to provide a similar experience, but with a lot more zombies and a biker theme.