An admirable answer my dear fellow.
Subjectivity I agree works with “Fan-fiction”, as writer’s personal feelings or flavours are vastly diverse in strength when writing up as you said “compelling and complex traits” characters.
Your answer shows light on subjective writing whiles understanding concept of constructing an imaginary world. To play with certain interaction throughout the story.
Suits well with your ideology wanting to be free.
“A real answer exist not for being real, but for being believable as well.” Holy shoot I actually typed my first quote.
Although if I had to answer, it’d be an objective one that makes believability in stories possible.
First impression I consider highly important through any storytelling as the writer promises their readers of what fan-fiction they’re getting into.
Subverting expectations is another case but nailing the execution allows the potential of characters, world building, plots, concepts, and much more to flow well.
Whether these promises actually are kept or failing to meet them up to the writer.
I’m no professional nor genius in literature.
This is simply from experience from reading many light novels/books.
Take this however you want.