Tell me if you have not heard this story told a million times before. A hero is called into battle through a prophecy of some kind. Despite some hesitation on the hero's part about their newfound role at first, they eventually accept it and go onward in their adventure. They meet a ragtag group of characters who usually are defined by a single personality trait. Villains of the week attack our heroes along the way but the heroes almost always end up victorious not counting one of two major character deaths. The final boss is met at the climax of the story with some plot changing information such as being the hero's father or sibling. The main hero will either fight alone or with a group of friends despite this barrier against the bad guy. Unless the story takes place in a dystopian future or the heroes are labeled as anti-heroes, they survive and win the fight. A brief celebration of the win takes place before the hero for some reason goes back to their life before the adventure.
This beta storyline for most fantasy novels is just one example of a plot formula. For those who do not know what a plot formula is, basically it is a plot outline for what is expected to happen in the story event after event. Lots of popular manga, comics, and popular TV shows also use plot formulas, usually to make the writing process less strenuous and keep an existent status flow. While these plots if done right can work great, a formulaic storyline makes many of the events that are to unfold extremely predictable. Predictability is never a good trait to have in a storyline, especially if you want your novel to become popular.
An example of a book that had great potential but got ruined because of a predictable plot formula is the supernatural romance book Nightshade. While creating a strong family dynamic with its werewolves as well as giving them a unique reason for their shape-shifting abilities, it is hindered by the formulaic love story plot line. The forbidden romance of main character Calla to the "completely ordinary" human character Shay and then the creation of a love triangle with werewolf alpha Ren, who she was promised to through marriage, is nothing new from other similar stories before it. The love triangle dominates everything in the story, which is very unfortunate. Without the love triangle, the story may have been great. The plot formula ended up killing a possibly great idea.
Warm Bodies like Nightshade had great potential for a story as well. The idea of human kindness being the key to curing zombies is creative all on its own. This novel and then film could have been a true winner if the idea of human kindness helping to save the world was the center focus. However, Warm Bodies ended up being overshadowed by many other great fantasy novels for using not one, but two plot formulas. Like most fantasy novels these days for some reason, main zombie character R has to have this romance with this female power symbol character Julie. Additionally, many of the zombie outbreak storyline tropes end up overtaking the plot at numerous times throughout the story. Again, Warm Bodies had great potential but two simple plot formulas ruined it.
Without mentioning a million other fantasy novels that fall into the problem of plot formulas, let me just say that in order to spice up the story you need to surprise the audience. Instead of a supernatural romance between a human and a werewolf, how about focusing in on the family dynamics between a pack of werewolves? Instead of another zombie outbreak story, how about making a fantasy novel focusing on the power of human kindness? There are a million ways to go with a single storyline. Heck, even a series can be randomized enough story wise in order to keep things fresh for many novels. Whatever you decide, remember this. It is better to color a picture right outside the lines than it is to color inside of it.