Like, it's not weird for wholly original romance novels to have otherwise disposable friend characters who act as author mouthpieces at crucial points in the story. (It's also not uncommon for romance novel characters to have NO close friends or family, in my experience, which is a different thing.) Former fanfics do not even slightly have a monopoly on, "Hey, what's up with this weird friend character? They seem a little out-of-place here? Also seems like you're maybe under-utilizing the weird, comedic relief friend, tbh; they could be doing more in this story."

Audience familiarity with (and affection for) the characters and their relationships to one another can be one of fanfiction's strengths re: storytelling. You can play with a lot of reader expectations this way.

Anyway, if I was reading a Coffee Shop AU Star Wars Reylo fanfiction (you could basically use this example fic to ward me personally off like garlic against a vampire, tbh, but let's pretend)... even if Finn had nothing to do as a character in this story besides be Rey's coworker at the coffee shop, I would be even more suspicious and dismayed if Finn was completely missing. We EXPECT to see at least briefly all of the major members of a cast in an AU fanfic of certain length, barring unusual circumstances.

Even if he is almost completely irrelevant to the meet-cute at hand, I WANT to know where Chewbacca is in this AU! I want to enjoy a little chuckle when, idk, C-3PO is turned into a high-strung, middle-aged office guy with either a short, non-English-speaking husband or a Border Collie-Corgi mix dog named Artoo that runs his entire life. It's delightful to see the characters I know and love in the background!

But when you file the serial numbers off of AU fanfiction, a character's role in a story can no longer be Being That Character. Now, you potentially have a bunch of story elements that conflict slightly or conflict very seriously. Now, you may have additional characters whose presence is no longer perfectly explained by the fact that you are expected to include all the Avengers in your MCU fanfic.

Now, Not-Chewbacca's quirks cannot be explained away by him Being Chewbacca. And suddenly, readers are like, "Hey, kind of weird throwaway detail for Not-Kylo's dad to be a former drug runner who lives in a van with an old, foreign war vet! Is that going to be relevant later???" Once you cut the fanfiction strings, your new story may feel riddled with inconsistencies and missed opportunities. Why are these characters here if they're 1) not contributing to the plot, 2) not contributing to character ans relationship development, and 3) not even really contributing tonally (humor, angst, fluff) to the story?

He did a wonderful job, but as he was returning my plant to my custody, he explained "I watered her every other day, I gave her new dirt, moved her into the sunshine, prayed the rosary-"

"What?"

He looked at me, confused. "Isn't she a plant you pray with?"

"No," I said slowly. "She's a prayer plant. That's the... colloquial name of the plant."

Upon seeing his stricken face, I realized something went wrong. Turns out, this entire time, he's been including my plant in on his prayers and leaving her near icons.

Behold, Dorothea

He turned to me, sighed, and said in a very deadpan tone-- "Plants don't have covenants, Joy,"

Not only that the language itself sounds straight out of a marvel forcibly funny haha moment, but Menelaus? The Menelaus who all thoughout human history, from Homer, to Sappho, to Euripides, to Virgil, to Shakespeare, to Brooke, to Barth; from Classical vases to Medieval poetry to Reinessance art, has been known as the man whose sword fell from his hands at the sight of his wife after 10 years apart?

"What joy for me to hold my wife" Menelaus?

"He flung the sword away, and kissed her feet" Menelaus?

"I believe all. I understand nothing. I love you" Menelaus?

"I have no fault to find; my wife, he daughter of Zeus and Leda, is mine again, she whom her brothers on their snow-white steeds, whilst torches blazed, made my happy bride, but gods removed her from my home." Menelaus?

The Menelaus who was picked out of the suitors by Helen herself? The Menelaus who couldn't kill a begging Trojan so his brother had to do it for him? The Menelaus who rescued Odysseus and got out Patroclus' body? The Menelaus who volounteered to fight Paris one on one so everybody could go home? Who kept expressing his guilt about himself being the cause of war through every assembly he spoke at in the Iliad? Who after 10 years away at Troy, took his wife through a 7-year long journey at sea back home, where he reinstated her as the queen of Sparta? Who wept like a child in front of Telemachus recalling the war and his brother and all the friends he lost and the only consolation being his wife' gentle hand on his?

You want me to believe that man scarred Helen's face and called her only worth half the ships because of it?

Me: