If it’s done badly, that’s a Possession Sue. Not sure if we have a trope that covers it being done well.
Edited by SapphireBlue on Aug 21st 2020 at 8:34:39 AM
I'd say no. According to the trope descritiption, O.C. Stand-in is basically a fanfic equivalent of Ascended Extra, in which a fanfic expands the role and characterization of an otherwise background character. It's called "OC Stand-in" because the character in question lacks any trait that, aside from the name and appearance, the writer must write everything else about the character from scratch—just the way they would if they were writing an OC.
By that definition, major characters cannot qualify, as they already have established traits, personality, goals, etc. for the writer to use. However, even if the fanfic writer decides to ignore all of those traits in their writing and re-create their personality from scratch in order to make them a better Author Avatar or Audience Surrogate or something, then it would probably fall under Adaptational Personality Change or similar trope.
I just found Out of Character though.
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O.C. Stand-in is for when fanfiction writers use minor characters from canon as a self-insert or an Original Character. However, I've seen fans do the same thing (turning a canon character into a self-insert/OC) for main characters. This is common in works where the protagonist is an Audience Surrogate (i.e. Harry Potter, Bella Swan). So, if a main character is used this way by fans, could they be considered an O.C. Stand-in?