: In a series of books, what happens after the coming of age? I'm writing a novel focusing on a single character POV. For many aspects, it can be considered a coming of age story; along with

If the first Hero's journey is more or less completed by the end of the first volume, you can make him/her the Threshold Guardian for someone else making his or her own journey in the second volume. This allows the books to be connected, but you don't have to strive to find ways for the main character from the first book to grow.
Seriously, this is how things work in real life. Your own parents are your Threshold Guardians, and if you have children and are anything remotely resembling a good parent you will fulfill the same role for them.

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A person's growth as a person does not end when they come of age. I am a very different person now (at 53) than I was at 21, to the point that I am abjectly ashamed of my younger self.

It is true that when a person comes of age, they no longer have to deal with the struggles of a person who is coming of age. Once you've gotten the girl or guy of your dreams (a common ambition in coming-of-age tales), you no longer need to get her or him; you move on to weightier things, for which the earlier struggles prepare you.

We also find (at times) that the steps we took to overcome a difficulty in an earlier stage turn out to be less effective than we had hoped, or—even worse—have compromised our ability to deal with a more important problem later. What seemed at the time to have been a wise and reasonable expedient turns out to be a fatal error, and the hero must learn to stop doing what is easy and start doing what is right.

It's difficult for us to appreciate it now, but once upon a time sequels were rare in speculative fiction. Galatic Patrol, the first novel in the acclaimed Lensmen series, ends with Kinnison decisively beating Helmuth, the head of a galactic criminal/military empire. It was a surprise to many readers when in the opening chapter of Gray Lensman, we discover that Helmuth was only a minor flunky in a larger organization. (The Lensman series is also an arms race gone mad, but that's another topic.)

Addendum

You can follow up the coming-of-age novel, and take the character's growth and path in a new direction, by constructing a Hero's Journey in which the coming-of-age portion you've already written becomes the mere beginning of the Journey. You can do this even if the first volume is a complete Hero's Journey in its own right, but you have to be very sure that this time, the features and challenges of this journey are different in kind and not merely in degree:

If the struggles of the first volume are met by being clever, in the next stage the struggles are such that no humanly-possible degree of cleverness will suffice; instead the Hero must grow in a virtue that is more fundamentally important than cleverness and perhaps is even antithetical to it (such as honesty or humility).
The dangers are not merely physical, and the struggle is not limited to the physical realm. The ideas of what constitutes good and evil were accepted without question in the first stage; now they need to be checked. The Hero's being on the right side of the struggle can no longer be taken for granted, but instead is something that has to be held to a real test of truth.
Friends become enemies and enemies become friends.

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In a series, I expect to find one of three approaches:

a) One single plot that covers the entire series and which is divided into smaller parts in order to give each book some level of closure

b) One plot that covers the entire series, and smaller plots that start and end in each book.

c) small plots that start and end in each book, with the unifying line of the series being a location or something else.

So the first step is to identify which approach you're following.

Approach A

The plot is about the MC's coming of age (becoming an adult). But one does not become an adult as a result of a single experience. Perhaps the MC started out as insecure and found confidence, but there is still a lot to learn in order to become a full fledged adult. The next book will simply be another step in that long travel.

Approach B

In this case, there is a major plot that requires the MC to come of age in order to successfully tackle it. So it makes sense for each book to have a slightly different tone, though not necessarily. The attitudes of the MC can change, as her worldview, but the tone doesn't have to become grim.

Approach C

From your words...

the plot does not end with the current novel. A major arc is resolved, but many themes are left open for a potential sequel.

...I assume 'approach C' is your situation.

If the readers are after 'coming of age' themes, then there's only one possibility: turn this into option A and simply say it's the next step in her growth. This way, each book tackles one difficulty in the process of becoming an adult: responsibility for one's own actions, developing skills essential to an independent life, romance and sex, financial difficulties, dealing with a job/boss... anything that fits the setting and separates a dependent, inexperencied youth from an independent, experienced adult.

But the series can be presented as being something else. This could be the adventures of a woman, with the first book explaining how she became that independent woman. In that case, the reader knows that the first book is simply the setting to the real story in the upcoming titles.

In my understanding, people purchasing book 2 of a series do that because they're happy to read the follow up to book 1.

I completely agree with you.

Yet, leaving behind the "coming of age" theme can be considered as breaking a premise.

So do not present 'coming of age' as the premise of the series. It's simply the first theme of the series, with other themes showing up in later titles. If you do not trick your readers by letting them think this is all about growing up, they'll carry on with you.

What happens after the classical coming of age story?

In Historical Fiction, it's not uncommon for the main character to start out as naïve and then grow up into a powerful - often cunning - person. The difference is that the focus isn't in the coming of age. The focus is in finding out how the character got to the top, and then how they faced their enemies in order to remain at the top... and whether they were successful or not.

Most stories marketed as 'coming of age' tend to avoid this wide focus. The character faces the challenges successfully and becomes a grown man/woman. The End.

Can you rule out the "growing character" aspect without losing and alienating a part of your audience?

Most readers (IMO) who gravitate to these tales are after that victory and don't really care about the later adventures. They want to experience those growing pains and bask in the final victory. If that is the target reader (typically a teen or young adult), then there really aren't many alternatives: either stretch the 'coming of age' and explore all the themes you can find within it (which are plenty, to be honest) or don't have a sequel.

But if you are after a different audience, an audience who is fed up with 'coming of age' and who scoff at the 'happily ever after' finale, then pitch the 'coming of age' as simply the first adventure and give plenty of warning that things will carry on with other themes. Or avoid starting out with the coming of age. Let the readers get to know and care for the skilled adult character before they have to put up with a clueless kid learning the ropes.

PS: I almost didn't write this, but then decided to give my personal take on the topic.

I'm terribly fed up with the clichéd 'coming of age'. I like the idea - I love it, really! - but it never goes beyond that first victory and the 'happily ever after'. A character that becomes adult and keeps on having adventures sounds far more interesting, though the narrator mustn't strike too much of a different tone (IMHO).

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People live their lives. Once they come of age people take up the responsibilities of their adult role in society. This need not represent a major shift in tone/style, "people grow or they die, or rather they grow until they die" sorry you'll have to look up who said it first but the principle is that people, real people, are always growing and changing. A child becoming an adult, even in a society with a defined age of adulthood, is not an instant event. Everyone has to learn their particular role in their society and develop the skills and relationships that make it possible to carry it out. That takes time and experiences great and small and exploring that process is the same in principle, though different in content, to the coming of age story.

For an alternate narrative remember that while heroes are usually people who only do extraordinary things in extraordinary times, (so when times are more normal so are they), they can also be trouble in normal times because they don't fit in having done extraordinary things.

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Coming of Age is about becoming an adult. This is often for young adults the transition to a sexualized person; being interested in sex and romance, knowing what it is about, perhaps experiencing sexual attraction for the first time.

Anthropologically speaking, we see the same story in apes and other animals: The young reach an age where they rebel against their parents. They want to strike out on their own. In humans, you will see this kind of rebellion starting about the 5th grade, often a year or two before sexual puberty begins. But grade school teachers know that somewhere between 4th and 5th grade (ages 10 or 11) is when the boys start "showing off", and the girls start "whisper circles".

But becoming an adult can be much larger than just becoming aware of one's sexual attributes, or discovering sex dreams or orgasms. It can involve new ways of thinking, like questioning the wisdom of one's elders. It can be learning to decide for yourself. It can mean doing things in secret and keeping secrets, it can be bonding with a group more strongly than with your parents or siblings. In short, it can be about becoming independent, and seeking experiences which you know your parents do not approve (e.g. smoking pot, watching pornography).

Coming of Age doesn't have to be a single experience covered in a book; that isn't how it works in real life. Humans are not actually physically done developing their brain until past the age of 22, and we all know there are plenty of people between the ages of 16 and 22 that are, to all appearances, sexually developed as adults, but still ruled by emotions and behaving impulsively and carelessly; NOT like sober adults (even if they are engaging in sexual acts, getting high and drunk, etc).

So whatever advancement toward adulthood you accomplished in the first book; there should be plenty of other advancements toward adulthood left in sequels. There is still a wild phase left to cover, that many people experience in the first years of college, when they are first out on their own with no parents to stop them, and legally allowed to do anything adults do. That is a whole new discovery phase, and it can be interesting to cover as well.

The "New Adult" genre follows the "Young Adult". The distinguishing feature of New Adult is that it appeals to an audience of roughly 18 to 26 or so; which is basically the age group of the single adult on their own, more or less before marriage/commitment but navigating their way toward that. In the USA at least, unlike the YA market, this often involves actual sex and sexual awareness (the average age of voluntary loss of virginity for women in the USA is age 16.5; for men it is age 17.5 (female virginity is typically lost with males older than the woman)).

There are also issues of responsibility; whether the character has a job or attends college, they have no parent to keep them on track, and college professors don't usually fulfill that role either. You show up to class or you don't, I will pass or fail you based on your tests, projects and homework. So they can get in trouble on failing to pay bills, speeding or driving drunk, sleeping through their alarm and failing to get to class, breaking rules at school or on the job, any number of things they can do wrong.

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It's rare, but not unheard of for a series to shift genres as it progresses --with Harry Potter perhaps being the most notable example. As the protagonists grow up, the style and content of the books shift to follow them. Far from being a detriment, this was arguably a key reason for the series' success. And, of course, we're used to seeing this in more autobiographically based fiction, such as the Little House series, which follows Laura from young childhood to adulthood.

With that said, there are challenges with doing this. I was a big fan of Anne McCaffery's Dragonsinger trilogy growing up, but found the shift from the more innocent, juvenile tone of the first book to the the more older teen-oriented themes of the sequels to be a bit rocky. Similarly, Virginia Hamilton's Justice trilogy, which genre-shifts from contemporary realism to science fantasy, struggled to find an audience, despite the author's fame. One solution I've seen used by authors such as Garth Nix, Diana Wynne Jones and L. Frank Baum is to keep the setting, and the coming-of-age themes, but to shift the focus to a new protagonist. C.S. Lewis also did this with the Narnia chronicles.

If you do want to follow the same protagonist forward, however, I think it's not a breach of promise to chart a new structure in the sequels, as long as it isn't too jarring a disconnect. In other words, each book can be a (somewhat!) new contract with the reader.

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