It's really that long? Holy crap.
I bet the writing quality really improves over the course of the fanfic, given how long it's been going, but that won't really help people who actually try to start reading since they'll still have to start from the beginning in order to understand the story. I tried to start reading it once and gave up because the writing in the early chapters was poor on all accounts: spelling and grammar, characterization, and plot.
If it's not the longest piece of literature ever written, it's definitely damn close. Over 3.5 million words and counting last I heard (which was May).
I don't know about the writing quality as I never really read it but apparently it does have a following and people do like it.
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As far as Smash Bros fanfics go I haven't really ever read any. I don't read fanfics in general (only occasionally, out of morbid curiosity or to make fun of with friends), and TBH I would reckon it's hard to find good ones of Smash Bros, considering who makes up the fanbase. I do recall running into one I liked on Tumblr though. It was a Rosalina and Meta Knight one, from Meta Knight's POV. It used the typical scenario of "Smash 4 is a tournament and the characters live in some sort of premises nearby the arena while the tournament is ongoing", and I believe it was part of a series of "character pair-ups/rivalries". This one was basically just Rosalina and Meta Knight having a bro to bro (or mama to bro?) conversation, talking about the stars, their own lives, the tournament. This all being a night before they were set to face each other. Both characters seemed well enough in character (mostly Meta Knight - they made Rosalina a bit too timid though, IIRC, though I could be remembering incorrectly), and Meta Knight seemed to be a nice blend of his game personality and anime personality, which I liked. Meta Knight ends up venting some insecurities regarding Kirby to Rosalina, since he's sort of supposed to be a father figure to him in this fanfic series (which he basically was in the anime), and Rosalina reassures him that Kirby will do fine, and that as long as you have faith in your child and do your best to raise them right, then they are bound to do you proud. Also Meta Knight's opinion of Rosalina changes over the course of the story. It starts with Meta Knight just kinda brushing her off and only talking to her out of politeness, and he assumes she's just some princess or whatever, until she starts departing her wisdom and realising she's not a princess (I thought that was a funny reference to the fandom) and then he starts feeling a powerful energy from her or something. By the end of it, he's just excited to fight her the next day, as he'd rather fight a worthy friend than someone he despises. It then skipped ahead a bit to the match in which Rosalina wins, though I recall that section literally just being the last paragraph.
As far as writing my own goes, I've written two non-Smash Bros fanfictions before, but that was years ago and I am embarrassed by them (one of them was written for someone though in order to cheer them up). I suppose I could write a good Smash Bros one if I wanted to, and I do have two ideas. One involving Rosalina and Mewtwo, and another involving Mario, Sonic, Mega Man, and Pac-Man. But I dunno, I likely won't ever bother.
