A complementary story instead of a sequel to the original game, The Last of Us: Part II will star Ellie as the main protagonist, and the game takes place when she is nineteen years old. More details will be released in the coming months after the PSX reveal.
According to Wikipedia:
"The Last of Us Part II is an upcoming action-adventure game developed by Naughty Dog and published by Sony Interactive Entertainment for the PlayStation 4. Set five years after the events of The Last of Us (2013), players control 19-year-old Ellie, who comes into conflict with a mysterious cult in a post-apocalyptic United States. The game contains survival horror elements and is played from the third-person perspective. Players can use firearms, improvised weapons, and stealth to defend against hostile humans and cannibalistic creatures infected by a mutated strain of the Cordyceps fungus. The game was announced in December 2016 and is scheduled for release on May 29, 2020.
Development of The Last of Us Part II began in 2014, soon after the release of The Last of Us Remastered. Troy Baker and Ashley Johnson reprise their roles as Joel and Ellie, respectively. Neil Druckmann reprises his positions as creative director and writer from The Last of Us however this time the story is also co-written by Halley Gross. Bruce Straley, game director on the original game, left Naughty Dog in 2017. Anthony Newman and Kurt Margenau were selected to be game directors for Part II; Newman was previously the melee combat designer for The Last of Us, and Margenau was game director on Uncharted: The Lost Legacy. Gustavo Santaolalla returns to compose and perform the score. Plans for multiplayer were cancelled because resources were shifted to improving the scale of the game. Naughty Dog stated that The Last of Us Part II was the longest game they had made."