No, it's not appropriate. Deku is a character with his own depth, goals, and story within the MHA universe. Fanfic should aim to explore his hero journey, his relationships in terms of mutual respect and support, not create content that focuses on inappropriate or disrespectful ideas like this.
I don't really have a list of specific 'mha deku x boobs fanfic' as it might lead to some inappropriate or objectifying content. Instead, if you like Deku, you could search for fanfics that focus on his growth as a hero, his friendships, and his challenges. For example, there are many fics that explore his relationship with All Might or his classmates in a positive and meaningful way.
Yes, there are quite a few. On Wattpad, you can find stories that explore the concept of Fem Deku as a secret singer in various ways. Some might focus on her journey in the music world while hiding her true identity, like how she overcomes challenges in the industry without revealing who she really is.
It could be a Muggle - born witch who he meets outside of Hogwarts during the summer holidays. She's not part of the typical wizarding society he's used to, so they keep it a secret as it's an unusual situation for a wizard like Harry to be involved with a Muggle - born in such a way.
The uniqueness of the concept. Combining Fem Deku with being a secret singer is something that stands out. It offers a different perspective on the character and adds an element of mystery.
There could be an original character created just for the fanfiction to be Harry's secret girlfriend. Writers often introduce new characters to add new plotlines and romantic interests for the main characters.
The main differences are in their expressions and some minor details of personality. In the manga, Deku might seem a bit more intense, while in the anime, he's sometimes presented a bit softer.
Another scenario could be that they start off as friends or classmates and gradually develop feelings for each other without realizing it at first. Then they keep it a secret because they are not sure how the other person really feels or they are afraid of ruining their friendship. In this case, there could be a lot of internal monologues and awkward moments between them in the fanfiction.
Well, maybe the story could be about how Harry meets this girl in a hidden corner of Hogwarts, perhaps in the Forbidden Forest during one of his adventures. They start to secretly exchange letters and meet in the dead of night. They keep their relationship under wraps because they fear how their friends and the wizarding world might react.
In 'deku meets future deku fanfiction', perhaps the future Deku warns the present one about upcoming threats. He could tell him about villains he'll face in the future and how to defeat them. This would give the present Deku an edge in his battles and also in his growth as a hero.