I already went off on this on Twitter, so I will be brief.
Mary Sue. Go read the TV Tropes essay, which is a good history of the term and its rather fuzzy meaning. But note some of the common factors. Special powers. Implausibly talented in an incredibly wide variety of areas. Remarkable physical traits. Everyone wanting to be with her. Closely related somehow to the writer’s favorite character. TOO PRECIOUS FOR THIS WORLD. Single-handedly saves the day while the canon characters watch dumbfounded. Obnoxious and everyone loves them anyway, even the antisocial characters. And so on. Oh, and is shittily written. Frankly, it’s a term that gets too widely applied as is; it’s become almost meaningless after migrating out of fanfiction and at this point reads as shorthand for “there is a canon character that I personally don’t like and think is too powerful so nyah.”
But if you want to redefine “Mary Sue” to mean “any young, untested character who has special powers, is the protagonist, and saves the day by developing said awesome powers” then every goddamn fucking character who has ever existed in a scifi or fantasy film and undergone the coming of age or chosen one narrative is a Mary Sue. The entire fucking point of the coming of age or chosen one narrative as it’s combined with the science fiction or fantasy genre is ordinary person proves to be extraordinary and goes on a journey to save civilization/the world/whatever.
You wanna bitch about Rey from The Force Awakens being a Mary Sue? You better also have already shit all over Luke Skywalker and Harry Potter, to name the top two offenders on the male side for having the same goddamn character arc*.
Or do you actually mean “any young, untested character who has special powers, is the protagonist, and saves the day by developing said awesome powers while being female“?
* And actually, I’d fight you anyway if you wanted to call Luke or Harry “Mary Sues” or “Gary Stus.”