In short, the last thing I want from The Last of Us 2 is for it too become episodic.
I don't want The Last of Us to be a thing were each instalment tells the story of a new set of people surviving in this "well realised world".
While I'm sure a Last Of Us game with new people could be a good game, it would not be a great game, and would fail to live up to the first one.
Could The Last Of Us work without Joel and Ellie?
As for the argument that The Last Of Us is about culture. Yes The Last Of Us shows different types of cultures that form after the outbreak, but it doesn't show a lot of the cultures that existed before the outbreak. Yeah sure they had a few movie posters here and there, and Ellie would ask some questions about what life was like before the infected, but this was never the focus of the game.
The Last Of Us is not an exploration, or representation of American culture (or any real culture).
Let's compare The Last Of Us to a game that actually does focus on American culture, Grand Theft Auto V.
Americans are obsessed with guns, violence, celebrities, social media, cars, money, etc. And all this stuff is not only prevalent, but is the focus of GTA5. Not so with The Last Of Us.
The Last Of Us is a great game, so people assume that, if it's such a good game it must be a meaningful look at different cultures, and our society as a hole, but this is not the case.
The Last Of Us is about something deep and meaningful, but it's not society.
The Last Of Us is a character study.
The Last Of Us introduces us to it's main character (Joel), and throughout the game we get to learn more about him, and what makes him tick. The Last Of Us is one of the only games that make the player really question who they're playing as. Joel is not the hero, he has done bad things in order to survive, and the game hints that he's killed innocent people, but Joel is not a villain either. throughout the game Joel's personality and motivations change. Joel learns to care for another human being (Ellie) and accepts her as his own daughter. Joel is neither hero nor villain, he is a man with complex needs and emotions. This is what lets us relate to him, and his relationship with Ellie. Joel is not some righteous super hero, he is just a man trying to survive, he doesn't always make the right decision and that's what makes him so relatable. The ability for the player to see themselves in Joel and Ellie, is the strength of The Last Of Us.
The story and the world of The Lat Of Us is not important. The Last Of Us is about how we can connect to it's central characters, and how they form this bond with each other.
Sure they can try to make the player feel a similar bond with new characters, but Joel and Ellie had such an impact, that any new characters would feel like second rate to the first ones. You may be able to make a character that's as bad ass as Joel, but you will not be able to make another Ellie.
Ellie brings a lot of personality to the game, that any new character may not have. You can try to introduce a new lead with a similar personality to Ellie, but it would just feel like they are trying to be Ellie, and the audience would wonder why you didn't just bring Ellie back.
I have stated before that The Last Of Us is more of a character study than anything else (a story in which the exploration of a central character's personality is more important than the plot). The Last Of Us gave us a look at Joel's character. The sequel should do the some thing with Ellie. It should show us what kind of a person she grows up to become, maybe not jumping to when she's in her 30s, but it could jump ahead a couple of years or so, to when she's around 18. Show us how her relationship with Joel grows, and how she has developed.
Joel's story may have come full circle (I will give you that), but Ellie's story has just begun.