I will go even further by analyzing the following situation:
It turns out that several years ago many writers asked ourselves the same thing in a Spanish forum, because like all of us, obviously our desire was not only to publish but also for those books to be sold and hopefully they would sell well. It was the time of
Twilight, The Hunger Games, 50 Shades, Harry Potter, World War Z... The enormous influence of the
Lord of the Rings, and all its clones and derivatives, due to the boost that
LOTR gave to the fantasy market , just as
The Hunger Games also revitalized science fiction, it was already ending; but we realized that, whether we liked it or not, the publishing market preferred authors who wrote about these topics. That is to say, it depended on what was fashionable on the screens to guide its editorial line. Which, of course, is still happening but to a lesser extent, I think mainly due to two aspects:
a) Script-writers crisis.
This produced a strange phenomenon, it made the production companies focus more on making remakes of already proven successes. In addition, the saturation of productions did not generate more money, it only made everyone get a smaller piece of cake. But the saturation also (it was to be expected that it would happen) had an echo in the publishing world. Some of them actually closed, or prestigious prizes disappeared, such as the UPC, in which renowned authors such as Robert J. Sawyer and others that I don't remember at this moment even used to participate.
b) Tendency to super productions with comic superheroes.
The Majesty of special effects. Getting bigger and bigger.
This produced another phenomenon, and made blog, forum, and fandom writers in general stare at each other, because until then you could write a romance-themed vampire novel and target it at the female market. You could also do one of zombies. You could even change the factors in the equation and see if it came out something Harry Potter-esque. But you couldn't compete with the special effects, further multiplied by the maelstrom of choreographies that leaves you breathless. In fact it can be done, we conclude. Or write, rather.
As long as, of course, we accept that the first thing is history.
That is the root of everything. So, since we depend so much on what we see on screen to write just a few lines, then I think we should also think like filmmakers and start with the concept or central idea first and only then worry about the characters and thirdly where the story will take place. But, thinking in literary terms, it is also good to do a "literary casting", for which we can ask ourselves things like:
For whom (the MC of course) living these experiences (the story) will mean an extraordinary adventure, or even a nightmare?
This, assuming it's a casting, helps define our hero's strengths and weaknesses. “We” will decide who this hero will be. It is our prerogative. But we don't have to overdo it either. Because at the same time we can also ask ourselves:
Who is the least qualified, guy or girl, to face this but he will have no choice but to do it?
This points to the conflict of the story, and although we have to accept, due to narrative needs, that certain "luck" of being close to the events that are narrated, and therefore having an importance that the reader grants us (for some reason is the one who is going to kick the bad guys and save the girl, or the other way around), we should not abuse or, worse still, brazenly resort to
Deux et machina. Here, I believe, is the key to a well-chosen MC, not because he is the smartest or most handsome, but precisely because he doesn't even imagine the tremendous mess we are going to get him into.
Why is only that guy or girl the only one who can carry out this mission or solve that riddle and not another?
This helps define what is special about the MC. But please let's try he doesn't be another orphan destined to be a great wizard or a lovesick vampire who, being what he is, is so cool that he doesn't kill humans, he just drinks animal blood.
Note that I have deliberately avoided talking about the antagonist. Because the villain may well be a cataclysm. Anyway he is the force that opposes the protagonist and therefore he is very powerful and difficult to defeat.
Now, regarding where this story will take place, I think we've already covered it at a reasonable length in other threads. But going back to the original question, IMHO what makes a story interesting for the reader, of course, apart from being well written, is that it provides a perspective that is somehow "new", either because of the idea itself itself or by the treatment or approach that we give to it. Some of course will say that there is nothing new under the Sun, but this is not so. Or put another way, the idea may not be original at all, but the perspective is.
For example, since we all seem to know more about cinema than literature, or we spend more time watching movies and series than reading books, Stanley Kubrick was one of the first to show the villains of the movie
Full Metal Jacket totally blurred and more like a distant rumor that according to the classic archetype of how a villain should be. In fact the only time he shows the face of one of those villains, what happens? Well, it turns out to be... a girl.
In
Dunkirk the villains are not seen either. Only the fighters appear bombing the poor boys on the beach. The only Germans that appear in the entire film, when Tom Hardy is taken away, are out of focus on screen. Clearer, water.
Sauron in the
Lord of the Rings isn't technically there either.
But, couldn't we also write a story in which the MC is also not seen and it is only known that he was there obviously for his good deeds such that he only appears at the end of the story and, of course, this way it solves the great mystery of who that was, and also such a noble person is one of the main attractions of the story?
That is an example of perspective.
On the other hand, in terms of treatment and approach, Tarantino has been giving us one example after another for decades, especially in relation to mimesis. That is, when the order of events is altered and, for example, it begins at the end of the story, continues through the middle and ends at the beginning.
In short, we can write the story we want. Another thing, yes, is that this one manages to sell, for which I suggest doing an analysis of the current film and television scene, that of video games and that of comics (I give up. Yes, there is nothing left but to accept that we are first television beings and in second order literary) and see what sells the most but, beware, paying special attention to how much useful life, or sales, remains for the topic on which we want to write.
For example, we all know that
Altered Carbon did not have, nor will it have, anywhere near the enormous success that the
Halo series, based on the video game of the same name, is having. So, if there are often skids even on the screen, we should be able to deduce why one series did not do so well and the other exhausted the entire stock of popcorn in the neighborhood. Because that is directly related to the script-writers. Therefore, we can transfer these conclusions to our stories, especially in regard to the handling of the plots. Which of course makes it clear that once we know what kind of story we want to write, and once the cast and the place have been chosen, the next thing to do is start planning the plot. Especially if there will be parallel plots.