TL:DR

Series like Battlestar Galactica and Game of Thrones are incredibly successful with pretty much everyone, not just the usual crowd of sci-fi and fantasy fans, because (among other reasons) they are first and foremost stories about human drama and only use the sci-fi/fantasy stuff to support the core themes of the stories they present. Most of the best stories we love from novels, movies and TV are dramatic first and "genre specific" later.

It's all about the drama

Be they aware of this or not, the majority of people consuming stories (novels, movies, tv series, etc) care about them because of the way they showcase human drama, where "drama" means the challenges and struggles unique to the human condition. The personal stuff. The visceral stuff.
Not exclusively.
Not all in the same way.
And a crap execution is crap no matter what it focuses on.
But look at any media that manages to grab its audience by the gut and yank it into its narrative, and you will find that "saving the world" is often something that happens on the background, it's the excuse to put the protagonists through a grinder of personal intimate struggle. Most great novels and movies and games have characters do stuff (the plot) mainly in service of having them confront their inner demons (the theme). When pull comes to shove, when things are difficult and have serious consequences, protagonist characters will toil and suffer and sacrifice and endure and overcome because to them, in a very private and special way, it matters. The most powerful motivations are internal and always trump the external ones. These are the reasons that lead good people to corruption, bad people to redemption, common people to heroism, etc. The narrative structure goes a little bit like this:

1. the protagonist holds a core belief
2. that belief is wrong
3. going anywhere near this is hard/painful for some reason (the inner demons)
4. the PLOT happens so that the character can, through many tests and challenges, eventually face their inner demons
5. the truth is revealed and the protagonist (along with their core belief) changes

If we were writers, doing this properly would mean spending a lot of time and effort on carefully crafting each character, their inner workings, the external pressures that force them to change. It is the hard work of creating a convincing and gripping evolution arc, plus all the other characters and places and circumstances supporting it. It's not an easy job for a professional writer, so it can't reasonably be what random RPG Players need to to in order to play a game. Or can it?

The traditional approach is to... nothing. Plot/action drives everything and Players have to somehow inject personal drama into the game on their own. If they ever think about doing such a thing. And if they have the skills to do it. And if it doesn't clash with other Players' expectations for the game. Some modern games, such as Mouse Guard, tackle the issue by throwing lots of effort and hard work at it; it's better than nothing, but it's also a less than ideal approach.

Emergent Story ≠ Character Arc

The use of a game structure that produces an emergent story can be a great boon and  a fertile starting point. No one at the table has pre-written or pre-imagined or pre-decided the story, and no individual participant can single-handedly impose a direction to it. The story emerges spontaneously, scene by scene, beat by beat, character choice by character choice, during active play. The GM can focus on active play rather than preparation, and everyone at the table will love the emerging story because for them, in that moment, it feels awesome and fun,  uniquely tailored to touch on their current interests, hopes, dreams, fears. To them, right now, it's great ... even if from a narrative/writing point of view it might look like burning trash :P

But this alone is still not a character arc. The overall story itself might, if properly directed by Players and/or mechanics, develop into an arc ... but the characters inside it might still fail to change, to face inner demons, to touch on human themes and drama. Most PbtA games are like this. PLOT gets in the forefront while THEME stays in the background. It's there, it's important, but it takes second place. Dungeon World and Monster of the Week are prime examples, with the very Apocalypse World falling very close to this paradigm.

Prime Time Adventures is a great example of a game that channels emergent storytelling through a structure specifically focused on character arcs. But due to its very direct approach it shares some of the same problems as Mouse Guard, and overall it really feels more like writing together a TV series than playing a (fantasy or otherwise) adventure. This being the aim of the game, it's not a problem. But for my very personal version of a fantasy adventure game I want something different.

Emergent Character Introspection

Fantasy World's solution to the problem is not entirely unheard of. The Hillfolk RPG and its "dramatic poles" present a very similar structure. But I find FW implementation both more incisive and easier to use for the actual Players at the table. The core trick is to not only make the story/plot emergent, but to also to make character/theme emergent, leveraging the intrinsic length of campaign play to bring it all out bit by bit. Rather than asking Players very tough questions out of the blue (like MG and PTA, and Hillfolk too) Fantasy World directs Player attention towards seemingly innocuous, practical, concrete elements that are useful to play their Protagonists right here and now. Only later, through continued active play, the game mechanics will gently steer Players towards discovering what they care about, what their core beliefs are, what holds them back, what their inner demons are, etc.

      Reward & Advancement

In FW you Level Up only at the end of the session by using a specific End of Session move.
This move guides the whole group, one Player at a time, through a step by step process that has them briefly reminisce about what happened during the session just concluded. The focus is first and foremost on the Protagonist's Issue and Doubt, and only partially on cool and fun action (which provides Expedience points). Issue and Doubt are the things that, if you engage with them, provide serious advancement.

Advancement in FW can happen in three ways.
You level up when you write the 3rd mark on your Issue track.
You level up when you write the 3rd mark on your Doubt track.
You level up when you write the 3rd mark on your Expedience track.

Without getting into details (you can just read the End of Session move in the wiki rulebook) the trick here is that Issue and Doubt can make you level up fast and consistently IF you play to them. Or against them. Or fulfil them. Or reject them. On the other hand Expedience points are earned inconsistently (mostly by rolling 1-6) and are spent fluidly during the whole length of the game session to fuel moves and special effects; in the end their use for advancement purposes is quite expensive and slow.

The result is that Players play with no great worries during the session, but at the end of it are led to put thought into Issue and Doubt. It's a way to train, rather than force, the desired behaviour.

      Weak Start -> Stronger Iteration

Issue and Doubt are critical elements of the emergent story, influencing its direction and focus points towards something that, independently of the overall action plot, will start, develop and be resolved ... a thematic arc unique of and personal to each Protagonist. So they MUST be perfect from the get go, right?
Well... no.
It is difficult and often unpleasant to create a character from scratch and be immediately asked to make critical decisions about it's future development, about it's imagined identity and personality, about it's yet unknown past life. So instead the Protagonist's creation process focuses on the here and now, asks provocative questions, and overall contents itself with putting together a Fellowship of Protagonists that make sense together. The rest will come through actual play.

Issue and Doubt are created now, with the aid of a simple step by step process. If the results are weak, vague, or don't fit what the Player hoped for... it will become evident in play and will then be easily fixed. Part of the End of Session move is specifically dedicated to the incremental error correction of these elements. This accessibility, ease of use and absence of pressure to perform perfectly on the first (blind) attempt are probably the main differences between FW's implementation of this concept and the ones found in other games.

If Issue and Doubt are mostly ok, there is always room for a bit of refinement.
If they are flawed or ineffective, there is a reward for coming up with new ones.
If they are fulfilled or exhausted, there is a reward for ending them and starting fresh.
All of these evaluations happen at the end of every session; as play evolves and more details get into focus and sediment on top of each other, thematic choices become easier and clearer.

      Fantasy Adventuring... with guts

All of this is meant to fill what I perceive as a void in the RPG hobby.
Games like Monsterhearts come very close to my idea of dramatic play but... well its not adventuring, fantasy or otherwise. Most adventuring games like Dungeon World, as mentioned before, focus on action action action and offer only marginal tools and attention to dramatic play. Other games try to approach dramatic play but, at least in my opinion, end up being unwieldy, ineffective, or fall short in some major way.

Fantasy World tries to offer long term fantasy adventuring driven by dramatic play. Elements such as Common Moves and Class Moves and World Reactions all work in synergy to promote setting exploration, character introspection and emergent plot play, while the whole reward/advancement system of Issue and Doubt shapes emergent character evolution arcs. All with the minimum possible effort required by World and Players, because I'm a horribly lazy gamer.