When writing the story and introducing the readers/viewers/players to the fictional universe, you should keep these percentages in mind.
40%-50% Established Canon: Meaning some things about the world should be confirmed, set in stone, and non-negotiable. These are the basics on which the entire world operates, and the things which can’t be easily altered, removed, or ignored. The main setting of your story should be detailed and realized, established.
The percentage here is a bit misleading. A good fictional world could potentially be expanded upon endlessly, way past what’s established in canon. The 40%-50% is about the core concepts of the world, the things that should be present in any addition or theory.
25% Open to Interpretation: Theories are things that are relevant to the main story, but are open to interpretation. The identity of a certain character, the minutiae of how a certain ability works or a possible twist or reveal in the story. A good world should keep at least something open to the imagination, even if only for a little while before additional information confirms or denies the theory.
Too many fictional universes these days are too open to interpretation in my opinion, and leave too much for theories to explain. It creates a great boost in publicity when people argue and talk about your fictional universe. But it leaves the world itself less rigid. The world becomes a platform for people’s headcanons and theories, rather than a realized universe on its own.
25% Open for Expansion: Expanding a canon is different than making theories. Additions made to the canon by fans don’t alter or recontextualize the canon material. A good world should be at least a little open to fans’ addition to it. Or even the additions of future writers who might work on the fictional universe you established. A world that can be expanded upon and added to without being ruined will be a long-lasting fictional universe.
There’s a reason they’re still making new superheroes and expanding on the Marvel and DC universes after 80 years.