Often, the theme of growth is present. Deku is constantly growing as a hero, and Midnight could play a role in that growth, either by inspiring him or directly training him. And there could be a theme of the unlikely pair, as they are quite different in age and personality, which makes for an interesting dynamic in the fanfics.
One common theme is the power dynamic. Given that Midnight is an established pro - hero and Deku is an up - and - coming hero, there can be a lot of exploration of how power plays into their relationship. Another theme could be mentorship turned into love. If Midnight was initially Deku's mentor in some way, the story could be about how that relationship evolves into something more intimate.
One popular Midnight x Deku fanfic might be 'Midnight's Guidance'. In this fanfic, Midnight could be portrayed as a mentor figure to Deku, guiding him in ways that are different from his regular teachers at UA. She might use her unique quirk knowledge to help Deku enhance his own quirk.
One aspect of deku and midnight love fanfiction could be exploring their relationship in an AU (alternate universe). Maybe in a world where Deku isn't just a student but an equal hero to Midnight, and their relationship develops from mutual respect to something more romantic. They might go on adventures together and discover new feelings along the way.
Some Tumblr blogs also focus on My Hero Academia fanfics. Look for ones that are dedicated to Deku or Midnight. They may have curated lists or even their own original 'Deku x Midnight' fanfics. It's a matter of exploring different blogs until you find something interesting.
You can try Archive of Our Own (AO3). It has a large collection of fanfics, and you can easily search for Midnight x Deku fanfics using the search function. Just type in 'Midnight x Deku' and it'll show you relevant results.
He could be getting married to Ochaco Uraraka. They have a great friendship and there are many hints in the series that suggest a deeper connection between them. Their personalities complement each other well, with Deku's determination and Ochaco's cheerfulness.
Maybe a plot where they get married in a secret ceremony due to security reasons. After marriage, they discover new things about each other. Deku realizes Momo's insecurities about always being perfect in public. They work on building a more intimate and understanding relationship. They could also go on a honeymoon where they end up having to use their quirks to save a small island from a villain's attack.
One interesting plot could be their journey of building a family together. They might face challenges like how to balance their hero duties with family life. For example, they could struggle with taking care of a child while also being on call to save the city.