There's no story without ideas and no ideas without inspiration. I'd like to take some time to tell you where my writing ideas came from. Hopefully, it can help on your own creative journey if not, there's some good insight on the origins of Moon Girl.
'Write what you want to read.' Is a piece of writing advice I can't say enough. Enjoying what you are writing is the key thing to finding the motivation to keep going with a long story.
In order to put that into a writable idea though, sometimes we need to step back and think about what we enjoy. For me the most obvious thing is superheroes. At the time I started Moon Girl I'd not long been going out with my boyfriend who introduced me to the joys of the Arrowverse shows, Young Justice and educated me on the wonderfulness of the bat family. (Tim's cover name is Tim Drake because it's his favourite Robin). One of the reasons that Moon Girl exists is because I was interested to see what I could do with superheroes, what spin could I put on it.
'Write what you want to read.' is a bit deeper than simply the theme/genre of your story though. A long time ago, when I was in college I made a list of tropes I liked, things I really enjoyed seeing in books I read(haha books. I didn't really read books at the time. I exclusively read Fanfic). My tastes have changed since then but for the most part, knowing what ideas I liked to see the most helps me put together the bases of a story.
For Moon Girl, there are two tropes I liked that put together the core of the story. The first was badass female lead. Indeed it is a common trope but I don't really care, I love a female character that can take care of herself. A short girl that people probably underestimate, who can open a can of whoop-ass an any given moment.
The second is...I'm not sure if it's a trope but one of my favourite fanfics to read is when my favourite character either isn't what he thinks he is or is trying to hide the fact he isn't what everyone thinks he is. The Mulan approach I suppose, in the instance they know they are different at least. This is the idea that tends to be in all my stories in some form. I can't get enough of it, I love that dramatic reveal later on and all the other characters coming to terms with it and everyone accepting them despite their differences. It's wholesome goodness. :) (Unable to say where this comes into Moon Girl as it's spoilers for the second book but yeah, people not knowing what they are.)
Thinking about these things is defiantly a good start on building a world you will love enough to get through a first draft and edit the story because you have to keep in mind when writing a story you will have to read through it, a lot, and it helps if you enjoy your characters and world.
I hope it's given you some food for thought on your own project or at least you've found the Moon Girl triva gems interesting.
If your looking for more ideas on where to start building your story you can look at Playlist writing. Here's my blog post on it.