Every single Marvel or DC movie or show is fanfiction
My first impulse was to agree with you based on “Yes, any adaptation is a fanfiction”, but then I thought about it a bit more and I... gotta disagree in this particular instance.
Because yes, every single adaptation of something is essentially a fanfiction.
However, adaptations of Marvel and DC properties are... less so adaptations as that they are reimaginations.
That comes with the territory of these comics; because they are rebooted over and over and over again ad nauseam. A new run of the Avengers comics gives them a new canon, a new lineup, a new plotline. Same goes with every Avengers cartoon and with the Avengers movies.
The MCU never actually adapted the comics - they never took a specific storyline from the comics to actually follow it, it tweaked and twisted things, took from various elements and changed others completely... and created new canons. If the MCU wasn’t motion picture but drawn on pages, it’d literally just be the latest comic reboot of the Avengers.
The same goes with DC. The Arrowverse very much established the multiverse and established to share it with every single DC TV show and every single movie in a multiverse - we can only assume that all of the Earths from all of the comic runs also coexist in their own alternate realities.
A fanfiction is something that, in its nature, doesn’t affect canon.
Much like an adaptation. Sure, the PJO movies exist, but they will never affect the books. Sure, the PJO TV show will exist, but it is never going to affect the books.
The MCU and the Arrowverse are affecting canon. Tony Stark started to look more and more like RDJ after the MCU hit, artists leaned more into that - even though yes RDJ was cast to look like Tony, but it became a symbiotic exchange here. Samuel L. Jackson’s Nick Fury was such a blast that suddenly, Nick Fury became a black man, in cartoons and comics.
This exchange is what makes it special, for me, because a simple adaptation, a simple fanfiction, does not affect the source material; a book series will not change based on the TV show or movie made of it. The comics however acknowledge, or may even take into consideration - or flat-out adapt, considering the Arrow tie-in comics, that are official DC Comics - the canons created in shows, movies and cartoons.
Marvel and DC shows and movies are much more than just fanfiction or adaptations, they are simply a... different run and take on the characters and stories, in the exact same ways as it’d be if they were comics, because they have an effect on things, they leave their mark, they create their own source material.
Or, at least that’s my take on that. ^^
