It depends on how you like to write and what you are trying to do. The way I see it, writing anything good is not easy. However, there are different aspects that are hard with each.
Original novels require a lot of thought that goes into world building or setting. This includes a lot of things about how a fiction world works or what is happening in the setting you choose to make the story realistic. Here are a few:
is there magic and how does it work?
what are the laws of the society (real or fake) we are in?
what is the major conflict in the world during the story?
There are more, but this gives you an idea. All these things need to be decided at some level before you can even really start the story. The you can do whatever you want in that world rather easily.
Fan fiction stories already have a world with rules set up for them. This makes the things I mentioned above really easy. Everything works the same as the original story the fan fiction is based off of.
However, these stories are more difficult later when you have to make a plot and characters. The plot has to make sense in the world you choose without interfering with the original story. It can be hard to think of something to do if the main story covers all of the conflict from beginning to end.
Characters from the main story can be used as long as they are available (these characters can’t be in two places at once, so they can’t be in your storyline and the original one at the same time). This is difficult because you need to know so much detail about when and where things happen in the original story, but it’s also very hard to write someone else’s character correctly. If they are even slightly different from how they are in the main story, people will usually get upset.
Making up different characters is what I recommend. It makes it easier to write the fan fiction in general because you own the characters. However, you still need to do this carefully. There shouldn’t be any characters too similar to an existing character (clones are not fun to read), and the characters need to make sense in the world/setting of the main story. They need to follow the same basic social norms of the people in the original book based on what we know of those places.
For example, a rich character from a very poor country doesn’t make much sense. Even if there is one or two rich families in this theoretical country, they are probably important enough to be mentioned in the main story. It’s better to make a poor character form here so he can “fit in” when his back story is created.
Anyway, I hope this is helpful! Good luck writing.