Basically what every great writer should do when writing the characters is giving them a sense of autonomy. In this way, the characters become less as avatars for the author and more of a human being in the eyes of the reader. They especially become more human when their circumstances and conflicts have lots of gravity, in terms of how genuinely disadvantaged they are. For this reason, the "Song of Ice and Fire" series defined this decade because there are a lot of stakes put on every point-of-view character regardless of their noble status (or even because of their noble status).
What definitely made Shakespeare separate from his Elizabethan contemporaries was that although there were hints of pro-Anglican pro-Tudor propaganda in his plays, what really defined his work was that his characters had their own personalities. Even characters that were meant to represent stereotypes managed to entice sympathy from the audience.
Even the villains and anti-heroes serve a purpose in showing the human creature in his most rawest form, either through developing intricate rationalizations for their actions or highlighting the conditions that led them to their positions in the first place. Shakespeare especially kept these in mind when writing characters who are outsiders in their societies. Even when heroes are outsiders in the exact same ways the Shakespearean anti-heroes and villains are, their arcs become more realistic since the sources of the conflicts come from their own identities, to paraphrase the YouTuber Just Write.
Though realism has a limit before the gory and offensive shlock devolves the story into the level of grimdark. In this way, it is not enough to have "good" plots or characters, rather they need to be balanced among each other. Their values would need to be cohesively symbiotic and interdependent in order to have the story make sense.
The story must achieve a level of suspense that keeps the reader clutching the fringes. In order to do this, it is important to create subversions. That was what made George R. R. Martin's "Song of Ice and Fire" series so compelling, as it destroyed the readers' preconceived idea of fantasy.
But in order to have subversions, there would also have to have been conventions in place in the first place, which would lead to the creation of a canon. One of the issues that was discussed in my graduate Literary Theory course was whether the canon was important or not. In the post-modern criticism, the canon is seen as only favoring certain people while disadvantaging everyone else. In the case of the feminist Jane Tompkins in her essay "Masterpiece Theater: The Politics of Hawthorne's Literary Reputation," Nathaniel Hawthorne is only part of the literary canon because the literary authority at the time were just like Hawthorne--rich, white men; while the Marxist literary theorist John Guillory in "The Canon as Cultural Capital" argued that the canon only serves the purpose of establishing a national identity based on replacing the English literature with the declining religiosity meant to make citizens subservient to the state.
Based on these interpretations, the argument that is definitely made in the post-modern era is that the canon is representative of oppression. Though, Toni Morrison in her essay "Black Matter(s)" argued that American literature, just as well as American history, is shaped by conflict between white and African-Americans. She concluded her essay by asserting that this paradox creates a rich complexity to American literature.
The issue of time can change the value behind a literary work, whether when it is first published or one century afterwards. In his video about what type of music album would be considered a 10/10, Anthony Fantano, who refers music on his YouTube channel "theneedledrop," explained how a meaning behind giving an album a score would be subjective based on time. So relevance would be a major component of the value of a literary work.
In order to instill the relevance, there would be literary descendants that would spring from the inspirations of future writers. In the case of Tolkien, his "Lord of the Rings" series is considered great literature in party because of the authors he inspired such as Ursula Le Guin, Christopher Paolini, and George R. R. Martin. So a great story would be considered great by future generations.