I can't help with content, but ... this may be stating the obvious but keep narrative structure in mind. All memorable stories follow a pattern where complexity increases, rises to challenging levels, then a change occurs that reduces complexity, bringing us almost back to the beginning while some effects of the change persist.
A simple realization of this is the classic seven-part narrative, where the change is driven by the protagonist.If Marinette is your protagonist, i would start thinking what change she can undergo: e.g. becoming more confident, or more cautious, or readjusting her priorities such as realizing the value of friendships.
In a classic story she will undergo the change in response to an extreme crisis of some sort. Think about what scene could effect the change you want to describe. Now you need to construct some narrative leading up to the crisis.
Once that is done you basically have your work cut out for you. In a 7-part narrative your scenes would be:
- Opening: We see her in her normal life. This is important to establish a baseline. Explain the ground rules of your world, but you can also forshadow the twist, without giving the game away.
- Hook: Something is happening, it catches our interest. It could be just curiosity at this stage, or a threat on the horizon
- Early success: We start following the hook up and things go well, we are making progress. This is important to deepen the readers investment in the character.
- Turn for the worse: Only now do we see the the full scale of the problem, or there is something we overlook. Maybe we got tricked or betrayed and now we are headed into trouble.
- Crisis/Twist: In the moment of greatest crisis a change occurs in the protagonist and it is this change that allows them to overcome the crisis and emerge victorious.
- End: It's not over yet. Having survived the crisis we want to enjoy our newfound strength and we also want to see the protagonist go back to their normal lives. On the surface nothing has changed, yet everything has changed.
The above recipe basically gives you awesome fantasy stories. The majority of movies works like this (including everything that Disney has ever produced). Also once you have your plans laid out like this it is pretty clear what needs to go into the early scenes as there is a lot of setting up to do so that the eventual twist does not come out of the blue.
So we have a good story, but not a storygame yet. For a storygame you want to think about alternative endings. There is a temptation to have some 'wrong' choices that lead to quick deaths, but they are usually a let down. Instead you can have multiple endings that follow the same structure as above. For example there could be multiple ways to resolve the crisis, some of which may turn the the protagonist dead or evil (saving your friends, sacrificing yourself for your friends, abandoning your friends to save yourself, turning against your friends to become an evil superhero). Such evil bad endings can also make great experiences if you give them the same attention including the same sort of detailed ending (where the protagonist goes back to their normal life, now cackling softly to themselves).
Ideally we don't decide for an ending not in one big choice, but through a series of seemingly small choices, the proverbial slippery slope