Legitimate as in?
I assume you don't mean canon since that's pretty clear. Its not canon unless the author or whoever owns the IP says it is. It can be your personally preferred version of events but it's not canon no matter how well it fits and you should not be able to make cash from it without permission.
A legitimate form of writing? I mostly see it as practice writing or for fun TBH. You're building on existing things. You skip over a lot like the the world building and don't necessarily have to come up with all your own characters, tone, style of story ect. Then again I suppose being able to work with character and a world that isn't yours but still managing to keep the feel of the characters and world the same and keep the character consistent (instead of having them do what you would *want* them to do) is a skill on it's own. I still don't know if I would consider it equal to building your own world and characters but it's fun seeing multiple authors play around with same set of character and world can be interesting and fan fiction isn't inherently lower quality to anything original.
I assume you don't mean canon since that's pretty clear. Its not canon unless the author or whoever owns the IP says it is. It can be your personally preferred version of events but it's not canon no matter how well it fits and you should not be able to make cash from it without permission.
A legitimate form of writing? I mostly see it as practice writing or for fun TBH. You're building on existing things. You skip over a lot like the the world building and don't necessarily have to come up with all your own characters, tone, style of story ect. Then again I suppose being able to work with character and a world that isn't yours but still managing to keep the feel of the characters and world the same and keep the character consistent (instead of having them do what you would *want* them to do) is a skill on it's own. I still don't know if I would consider it equal to building your own world and characters but it's fun seeing multiple authors play around with same set of character and world can be interesting and fan fiction isn't inherently lower quality to anything original.