I can't believe I'm writing this. If you've not written a fan fiction before, I'm severely disappointed in you.
Yes, I believe that many writers dabble in fan fictions at some point before diving into making original works. Why? I'm one of them.
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To define it, fan fictions are content created by borrowing original characters and lores. If it is copyrighted and you did not create the world setting or characters from scratch, that is a fan fiction no matter how far-fetched that is. There are many kinds of fan fictions that I won't delve into because you don't need to follow any rules in creating fan content although there are many guidelines to abide by if you do not wish to start fan wars in the community.
Original work, on the other hand, is defined as a creation that does not previously exist. Yes, nothing is truly 'original' in this world. Every writer or content creator has some sort of reference somewhere. However, if the idea (despite the influence of references) is developed from a structure unique to the creator, I would consider that original work.
Qualities of Original Work:
Non-existing world building
Non-existing character lores
No reference to existing fiction works (referencing actual world events and myths is alright)
Character must be developed with direct reference to any existing fiction works
Events in the story have to be drafted by the creator and not borrowed from other fictional works
When an original creator reference to a different work they made previously, is that considered a fan fiction?
This is tricky. Technically, it is canon and original if there are no contradictory or inter lapping points with the previously created work. Many authors do that in multiverse settings, borrowing an existing world and building a new branch. However, if the reference is to write a completely different story that is not part of the previous work's content, it can be considered a fan fiction.
Alternate endings can be considered canon if published with the original work. However, they can also be considered a fan fiction made by the original author if that was the 'unpublished' draft with the original work. The line here gets really unclear so the author can decide if they made an original content with the reference or a fan fiction. Disclaimers and author notices are the safest way to define them.
How does it feel to write a fan fiction vs original work?
It's the difference between heaven and hell. The expectations are different. I won't say one is more enjoyable than the other but here are some points that you can check out.
Pros of Writing Fan Fictions
Skip the character building
Skip the world building
No pressure to edit or meet publishing deadlines
Can break writing rules
Able to deviate from sanity and cause OOC without repercussions
Not going to be stoned if a plot hole exists
Cons of Writing Fan Fictions
May start a fan war
Can feel inferior to other godly fan fiction writers
Can die from cringe reading what you wrote at a 3am insomnia high
Weeb confirmed, there's no denying it after you published it
Might not do the original work or creator justice
Pros of Writing Original Works
Full control over what you want to include or exclude (full character customisation is fun)
Able to decide how cringe you want your work to get
Enjoy the power high of the god in your works when you design something to break your readers completely
Able to murder characters without guilt (but you're a murder hobo confirmed)
Ownership bragging rights
Cons of Writing Original Works
Will be judged for terrible writing techniques and language
Will be stoned to death for plot holes
Have to commit till the end of the work
Deadlines
Have to meet high expectations of paying readers
Hating the things you created
In conclusion...
As long as you find something passionate to work on at the end of the day, I'd say that's good enough. There's no 'better option' in the world of arts. The only winner here is the person who has the most fun by the end of the day.