oh ive been thinking abt this lately
a type of fanfic im not prone to writing, but am prone to wanting to write really badly is 'same story, but edited to be Better and Less Dumb bc the POTENTIAL.' so a reimagining, like you said. i think its probably a really good exercise in storytelling - you get to observe a story, critique it, take it apart, and then put it together (hopefully) fully functional, learning a lot more about what makes it tick as a story, and how you can make something even better. the same concept, for me, behind adapting a film or novel into a comic - to get really intimate with the material and find all its ticking parts.
having a Complete Thing there makes things conceivably a lot easier - if you consider it a working first draft or skeleton for your ~transformative work. you can see it all laid out in hd, so flaws are easy to see and avoid (until you start going off into your own territory, i spose) - its kinda cheating.
in general for fic though, differences tend to be in how characters and world are set up - besides aus, most fic takes characters the audience already knows and puts them in a world the audience already knows, so they can just get moving with the action, no first act (well, setting up the scenario, but thats a lot less than scenario + characters + world thatd be done in an original work)