I think, like sci-fi, fantasy is pretty big on introspection and moral lessons, albeit with a more grandiose delivery. So it's best to choose a moral theme or question, and build outward from there to create a plot.
You don't have to think too hard about it though, since you're just starting. Just pick something that appeals to you, and if it gets more specific or changes later on, all well and good; that's what your first draft is for anyway.
I'll give some examples from some of my recent refurbished fantasy stories (i.e. aimless stories I made up when I was a kid, that I had to rework to make them...actually about something):
Story A: Cult Worship and Religion
What does religion mean to different people, how does it evolve and splinter and gain new meanings over time? How does it help; how does it hurt? How does it affect people when it does both simultaneously...? How does it define what it means to love, and to have value as a person? These are the questions that inspired this story, and are weaved into the characters' backstories and larger conflicts of the plot.
My main characters ended up being a cult survivor who ended up finding refuge in the more modern sects of that same religion, and his "apprentice", who also found refuge there for different reasons, and tries to balance his respect for his new guardians and the "peer pressure" to follow in their footsteps, with his growing understanding of who he is and what a religion is as a concept.
This is a slice-of-life fantasy story, so it's mostly about exploring those mindsets and feelings through character arcs, but it has a subtle throughline that focuses on slowly uncovering the cult survivor's secret history, until one of his old family members finally returns at the climax to bust the whole scandal wide open. And then from there we get the usual questioning of the self, "you're not that person anymore", all that good stuff~. ^^ Execution is king when it comes to slice-of-life; it doesn't need to be complicated.
Story B: Familial Bonds and Obedience
What do you owe to those who created you? What do you owe to those you create (if anything)? Where is the value in a disobedient act; where do you draw the line between meaningful protest and wanton chaos? How much are you willing to lose for the sake of independence...? How much are you willing to give up for the sake of stability, especially for those you love...? These are the questions that inspired this story, and are basically answered in different ways by almost every character in the fairly large cast. It's definitely a more traditional fantasy, all about learning spells and saving the world, with a musical flair to the worldbuilding and magic system.
My main characters ended up being a prince estranged from his abusive father, a princess born into a rebellion thanks to her loving but kinda emotionally-neglectful mother, and a princess who reluctantly decides to join that rebellion despite the pressure to "just stay out of it" from her extremely loving, doting, and overprotective father. In short, lotta issues there; too many to list right now. ^^;
The plot took the shape of a sort of Game of Thrones-style ensemble narrative, following all three characters' (and their families') personal lives and emotional moments as they get drawn deeper into the conflict. I always wanted this story to be devastating, something that forced the characters to pick sides and feel strongly about whether or not their loved ones supported them...so naturally, it centered around a revolutionary war, as the abusive father and emotionally-neglectful mother drag their children into a fight based on their ideas about what their Mother wanted for the world. It's parent-child dynamics all the way down~
Anyway, the one thing these two radically-different fantasy narratives have in common is the fact that the plot is tailored to the themes as I wanted to explore them. Introspection on religion and identity gets a calm, slow-moving story that allows for a lot of thought. Clashing ideas about respect and loyalty and the pressures of family gets a high-stakes, action-packed story that forces the characters to make decisions and act on them. Decide on a theme, ask questions about it, create characters that will ask those same questions, and craft a plot that will let them find the answers.
