One common theme is the struggle of the underprivileged. Since it's about Spider Man and homelessness, it often shows the difficulties that the homeless face in the city. Another theme could be the idea of hidden identities. Peter Parker already has his secret identity as Spider Man, but when he's homeless, he has another layer of secrecy to maintain.
One popular 'Spider Man Homeless Fanfiction' might be 'The Homeless Hero'. It shows Spider Man trying to help the homeless in the city while also dealing with his own superhero problems. Another could be 'Homeless by Day, Spider by Night' which delves into the idea of Peter Parker living a double life where his homelessness during the day is a secret he has to keep from his superhero identity.
Another interesting plot could be that Spider - Man accidentally gets involved in a legal case that She - Hulk is handling. Since she is a lawyer as well as a superhero. Maybe a case where a superhero's identity has been leaked and they are being sued. Spider - Man is worried about his own identity and She - Hulk has to figure out a way to protect him while also winning the case in court. This would mix the superhero action with the legal drama aspect that She - Hulk brings to the table.
There might be a passer - by. Maybe someone who just quickly walks by the homeless man without really seeing him as a person, which could be used to show the indifference of society.
One reason could be hunger. Since homeless people often lack regular access to food, their stomachs growl when they are empty. Another reason might be some sort of illness or digestive problem. Just like anyone else, they can have medical issues that affect their digestive system and cause the stomach to make noises.
Well, Spider - Man's agility and ability to stick to walls could be really useful in combat situations where She - Hulk is facing off against a more powerful foe. He could quickly move around the enemy, distracting them while She - Hulk delivers powerful blows. His webs could also be used to temporarily restrain the enemy or set up traps.
His spider - sense would be a very useful power in the MHA universe. Given that many battles in MHA involve sudden attacks and surprises, Spider - Man's ability to sense danger before it happens would give him an edge. He could use this to warn his MHA allies and prevent ambushes. Also, his super strength would make him a formidable force against the powerful villains in the MHA world. He could hold his own against even the strongest quirks, like All - Might's strength - based quirk.
The story of the homeless man and the piano might be one of hope and redemption. He might have had a difficult past, lost his home, and been living on the streets for a long time. But then he finds this piano. As he plays it, he pours out all his emotions - the pain, the longing, the small joys he has had. His music might touch the hearts of those who hear it, and they might start to see him not just as a homeless man, but as an artist. This could potentially change his life, as people might offer him opportunities to play in more formal settings, or help him get off the streets through their appreciation of his music.
I'd recommend starting with the most popular ones on websites like Archive of Our Own. They usually have a good summary that can give you an idea if it's the type of story you'll like.
There could be a fanfiction where Spider - Man and Spider - Gwen start to develop feelings for each other during a joint mission. They struggle with their identities and how a relationship between them would work, considering their superhero lives.