You can try looking on fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. They usually have a wide variety of fanfictions, including genderbend ones for 'leonardo watch'. Just use the search function on those sites and type in relevant keywords.
Perhaps a plot where the gender - bent Leonardo is the only female in a group of male characters, and there are some romantic sub - plots that develop because of this dynamic. For example, she might have a love - hate relationship with one of the male characters which adds an interesting layer to the story. Another idea could be that she discovers a new power or ability that is related to her being female, which was not present in the original male Leonardo's story.
I'm not sure specifically as there could be many interpretations. It might be a fan - made story that involves a character named Leonardo watching something related to a white object or person. Without more context, it's hard to say exactly.
Possible themes could include transformation. Maybe the 'x' related to white is something that has the power to transform Leonardo or his surroundings. It could also be a theme of isolation if Leonardo is the only one observing this 'x' and white element, and no one else in the story understands or is aware of it. Additionally, a theme of beauty might be present if the white in the fanfiction represents something aesthetically pleasing that Leonardo is drawn to watch.
To write a compelling Leonardo in fanfiction, first focus on his existing traits. He's brave, so show him facing his fears head - on in new and unexpected situations. For example, if he has a fear of heights, put him in a situation where he has to climb a tall building to save someone. Also, explore his relationships. Maybe he has a falling - out with a close friend and has to work hard to mend it, which shows his loyalty and determination.
For Natsu, if genderbended into a girl, she might have long pink hair that she ties up in a ponytail while fighting. Her clothes could be a more form - fitting version of the normal Natsu outfit, with a shorter skirt perhaps. She would still have the same fiery spirit, but it might be expressed in a more feminine way, like being more passionate about protecting her friends. As for Lucy, if she were a boy, he might have shorter blonde hair and a more muscular build. His outfits could be more like a male adventurer's, with a vest over a shirt and pants. His key holders would be more like a belt or a bandolier across his chest.
A really interesting leonardo x reader fanfiction to begin with is 'Reader in Leonardo's World'. It throws the reader character right into Leonardo's world, which is filled with all kinds of adventures. The author does a great job of making the reader feel like they are actually there, experiencing everything along with Leonardo.
Some other platforms that might have such fanfiction are FanFiction.net. However, the content there can be a bit more hit - or - miss. You might have to dig through a lot of different stories to find the good Raphael x Leonardo yaoi ones. Another option could be Tumblr. There are a lot of fan communities on Tumblr that share and discuss fanfiction, and you might be able to find some links or even full stories there.
Often, there is the theme of empowerment. Female Naruto in these fanfictions usually shows great strength and determination, breaking gender barriers in the ninja world. They might also have to deal with gender - based discrimination, which is another common theme. This discrimination then becomes a driving force for their growth and development.
Another theme is the adaptation of the characters' identities. A female version of Sam or Dean might have to deal with different expectations in the hunting world, which was mostly male - dominated in the original show. They might face discrimination or have to prove themselves more, which is a theme often explored in such fanfictions.