Well, there's 'Fusion of Powers' in the 'dabi x shigaraki fanfic' genre. This one is cool because it imagines what would happen if they combined their powers. It's a very creative take on their relationship. And 'Unexpected Comrades' is also worth reading. It shows their journey from being just acquaintances to something more, with some really heart - touching moments.
Another theme is the exploration of their differences. Shigaraki represents chaos and destruction while Deku represents hope and heroism. Fanfics usually explore how these two opposing forces interact and influence each other. For example, in some fanfics, they start to understand each other's perspectives better as they spend more time together.
You can try Archive of Our Own (AO3). It has a large collection of fanfics, including many 'dabi x shigaraki' ones. Just use the search function on the site.
One common theme is redemption. Often, the male reader is seen as a sort of guiding light for Shigaraki, leading him towards a path of redemption or at least a change in his ways. For example, in many stories, the male reader's kindness makes Shigaraki question his own violent actions.
Shigaraki is not dead in the manga. The storyline has many twists and turns, and his character plays a significant role, so his survival adds to the ongoing drama and suspense.
One great 'Shigaraki x Deku' fanfic is 'Unlikely Bonds' which explores a unique friendship-turned-something -more between them. It delves deep into their complex characters and how they might interact in different scenarios outside of the main plotline.