Let's talk about fanfic.

Yeah, I've done a bit of the first a decade ago or so. To me it feels different for me than writing a character someone else made up, because I don't think I know a character well enough after just reading (or watching) finished works featuring them.
And changing an existing character deliberately because I'd like them better that way just seems rude.

"I could have done better" or "X should have done Z"... yeah, what comes to my mind is the horrible "romance" and general impression of too much story in too little time in the first season of Legend of Korra. That's a big part of what led me to me not watching the second season, because why would I engage with something that frustrates me more than it entertains?
But mostly it's stuff like "drat, I'm really tired of the whole 'superpowered people protect a world that fears them' trope, I want to make up my own universe where people worked through that already". Or "I want to make up my own universe that (among other things) does not have a history of treating women as property". Not writing fixit fics or alternate universes for someone else's work.

"What if X happened?"... can't really think of an example, no, so those must have been very fleeting.

The problem I have is with people trying to dictate reality (or canon) to other people.

"How dare that author NOT like fanfic? It's a compliment!" when there's an entire subgenre, fixit fics, that are supposed to improve on the story, that is, it's not unreasonable to see it as the fanfic writer going "I can do better than you", which doesn't seem very complimentary.

"Granny Weatherwax MUST be a closeted lesbian, because she never had sex." Look, I get that being lesbian and seeing hardly any lesbian characters is frustrating, but I do not appreciate trying to erase one of the few characters I identify with because they explicitly say that she never had sex because other things were more interesting. (Presenting it as a possible interpretation rather than arguing it's the only one that makes sense would have been different.)

"Who doesn't come up with their own ending to open-ended series?" is a question I was asked the last time I talked about fanfiction, and it felt rhetoric to me. And, well, I don't do that.

And on the topic of francises and such, I dipped into the Star Wars expanded universe a bit, but gave up on it because of stuff that contradicted the original movies.
On the other hand, The Life and Times of Scrooge McDuck is in my opinion the best Disney comic ever, and it's basically Don Rosa making fancomics based on Carl Barks' work.

I don't believe fanfic is automatically bad, cause I've read some that was really well-written and interesting, great stories.

But I don't agree with the idea of fanfic as a universal interest or even need of humans, because people are different.

There's a great World of Warcraft Machinima out there of the doomed love between a Human and a Blood-elf that ends in a perfect storm of tears. And there were lots of comments along the lines of "Noo! He should have caught her!" Which is sticking a happy ending on a tragedy! Sometimes, the Little Mermaid has to die. Sometimes, Romeo and Juliet need to die in perfect misery. Sometimes, authors need to be complete and utter bastards to their characters. "Fixing" it so that Romeo and Juliet live and have lots of delicious sexy times is so missing the point.

I have to disagree that re-writing for a happy ending is missing the point. Lots of people have enough misery in their lives, that they want to read the happy ending, as opposed to the tragic. It hurts no one for someone to rewrite a tragic ending to a happy one.

So while yes, tragedy exists and sometimes, Romeo and Juliet need to die, Boy and Girl don't end up together, etc. - I see nothing wrong with someone wanting a happy ending for a particular pair and writing a story giving them that happy ending. Those who prefer the tragedy don't have to read it, they can remain satisfied with the original, while those who want/need a happy ending can have their wishes satisfied as well. No one loses, everyone wins.

I tend to write fanfic in shows that tend toward a formula where canon throws a curve ball which, logically, would make everything after no longer adhere to the formula only it DOES because they have to because Television. I'm reminded of RWD saying about one particular episode, in effect "We got it wrong. I know we got it wrong, that character NEVER would have done that, not ever, but because of actors and storylines and constraints, we told the story we were able to tell. But she never would have done that."

So the story I wrote took off from "She didn't do that" and I was far from alone, there is an entire subgenre of stories that start on that beach with her not doing that (or with her doing that, and exploring the part that television couldn't explore due to Reasons.)

It was fun. It was satisfying. It scratched an itch that show would have left FOREVER, I know, because I started an X-files fanfic over the same damn itch, ended up stopping writing for a long time, then 10 years later rewatched the series and hit that same stopping point due to that same itch and finished the story... and now I don't have that itch.

I didn't write it to tell anyone else what happened, I wrote it because the show created a logical inconsistency and my brain wanted to figure out what really would have happened, and then I immersed myself in that world (which is fantastic, btw) until a story popped out that scratched the itch. And if it takes 250,000+ words to scratch an itch, you better believe it's an itch that needs scratching.

Oh, and I don't think it's a universal need. There are whole series with huge fanfic followings that I don't write in because I don't feel the need. The show scratches the itch enough, or other writers do. I don't think other people have to like it. I don't even need other people to read or approve of my stuff, I write it for me, not them. It's one of the reasons why fanfic is easier--I'm absolutely not trying to please anyone but myself.