Terminology for this sub
Since the Minecraft franchise has no "canon lore" (for more details, see this document), this makes it somewhat difficult to use familiar terminology, including "headcanon", "canon", and so on. Therefore, in this post, I'd like to describe all the terms used in the theoretical community, along with their meanings, to avoid confusion and misunderstandings.
A theory is a collection of analysis and conclusions regarding an object in the Minecraft universe, conducted with the goal of finding the most plausible explanation for everything related to this "theory object". Simply put, you find an object in the Minecraft universe, analyze it and everything related to it in the Minecraft universe, and based on this, you draw the most plausible conclusion based on your analysis. That is, If, say, 10 facts point to conclusion A, and only 5 facts point to conclusion B, you choose conclusion A (provided that conclusion doesn't ignore any facts or contradict anything). When analyzing and creating a theory, you must use the entire Minecraft universe; all the nuances are described in more detail in the document linked above.
Headcanon/hypothesis is the same as a theory, but without any evidence or "most likely explanations." A hypothesis shouldn't contradict anything in the Minecraft universe, but nothing should prove it or point to it. Essentially, if you say, "My headcanon is that the Nameless One was called Herald while he was alive", nothing should point to that name specifically, but nothing should contradict that name either.
Canon - can only be used in the context of "this Mojang product is canon for Minecraft mythology" and means that something exists in the Minecraft universe. For example, "dragon egg is canon" would mean that dragon eggs exist in the Minecraft universe, while "lightning effects are non-canon" would mean that lightning effects don't exist in the Minecraft universe.
Fan lore/Fanon is your fiction that contradicts the Minecraft universe but is suitable, for example, as your "vanilla story".