Why does most Fanfiction feel like Mary Sue garbage, you may ask. Well that is because it is. The majority of times. At least for the ones with the most children, and the most weeaboo type otaku.
Take Highschool Dxd Fanfics for example. The majority of crossovers will not really be well written at all from a literary standpoint from someone who also indulges on copious amounts of varying genres of anime. Most will be about making Issei’s harem grow. And many of these will attempt to do so through NTR. No matter how much they tell you they hate it, if it’s Issei, there precious dreamt up harem king, then it’s great. Cuz he’s a pig by moral standards. And many of them are too. Wouldn’t be so bad if they admit that’s the reason they like him. Cuz you know. Being as assholian honest is the point of this place. And many times, if they try to use another series to give Issei a backstory, it will be straight up rip offs, and Gary Stu shit. Look at a certain One Punch Man x Dxd fic. Rather than making them unrelated and keeping it in character, Issei is the son of Saitama. Last I checked that sounds straight Gary Stu. Issei one punching his enemies. Seriously. They make long as stories that have no need for anything more than a few lines of Issei one punching someone. Even the average One Punch Man chapter takes less time to read than that.
Then of course, the dreaded Naruto fics. Those are just outrageous. So many that are about some shit AU meant to give Naruto a harem. Whether it be an in universe one or a crossover one.
We get that you worship Naruto. But that doesn’t mean everyone wants to do that as well. Stop polluting the net with this shit. If you want to make a story, good. If you want a harem in it, whatever. But wheb you make the crutch be that the Mc Stu you made will have a harem without even trying, then you might want to rethink your summary.