I know it can be hard to come up with a plot that is unique. But stealing someone else's idea and just replacing the OC with your own OC does not make it original. For instance, I've read quite a few fanfiction. Here are some cliches from different fandoms.

Naruto- Personally, Im not.into Naruto much anymore. But a few cliche's are soooo repetetive and get boring. Sure, if you write it well you can get away with a few, but please, be unique. Here are a few ive seen MULTIPLE times...

The Oc being in the same team as the main 3

The character being a girl disguised as a boy, for basically no reason

An oc being the top of the class, and being on a chunin level or higher DESPITE no previous training whatsoever.

Instantly understanding the meaning of Kakashi's test

These are just a few of the many cliches. Maybe I will add more to a later chapter.

Ouran Highschool Host Club- This group of hotties has a lot of cliche's as well. Being a good anime/manga, sometimes the fanfictions make things a little... Worse then before. Here are a few cliches.

The Oc being basically a second Haruhi.

If they aren't a second Haruhi, The host club decides to add a hostess program.

Also, all of the guys like her.

Oh, AND they continue to let her flirt with other people.

She has a dark past, involving a yandere or a crazy ex

She is a "commoner"

If not a "commoner" then a sibling of one of the boys.

The sibling hates her.

She is sent away by parents if she is siblings with someone.

She is related to Haruhi.

Must ALWAYS be a love story.

Again, not even all the cliche's.

Undertale- Yet another, Undertale. One of the most... Well. You know. Fandoms out there. It just makes you go "Ewwww what the hell?" While being a good game everyone hates it because of the fandom. Here are MANY cliches. (Might do a chapter on this one too)

Character is human

Character falls for sans

Character is depressed

Character has powers

Or a dark past.

Or both??!

Everyone loves her

Undyne wants to catch her

Again, almost always a girl, hence calling them "her"

Two words. Mary. Sues.

Stays in the underground for like, ever. And apparently Asgore, THE KING, WHO IS SUPPOSED TO KNOW EVERYTHING GOING ON, DOES NOT KNOW.

Saves monsters everywhere!

Maybe stops a genocidal Frisk.

Chara is evil.

Making friends with Flowey.

"Saving" Flowey.

Turning flowey back into Asriel somehow.

Sans remembering timelines.

Yeahhhhh... We need a whole chapter for this.

Another way to keep things unique, maybe dont follow the storyline of the show/game/manga? Make up your own with the same characters! ALSO! Make your characters a male! Thats about it for today, see you in the next chapter.

Note: If you guys have ANY questions, Please ask! I will make a chapter regarding your question. I will try my best to answer it!