Dabi could be a great character to pair with Tomura. They are both villains and their powers and personalities could create some interesting interactions. Maybe they could team up for a particularly difficult mission or have a power struggle within the League of Villains.
One idea could be exploring Tomura's past in more detail. Maybe show his childhood before he became a villain, how he was influenced by All For One. Another could be a story where Tomura tries to redeem himself, which would be a huge twist from his normal villain role.
In a crossover with the DC universe, the Joker could be an interesting character to pair with Tomura Shigaraki. Both are chaotic and have a disregard for life. Their meeting could lead to either a partnership in chaos or a battle for dominance in the criminal underworld.
One idea could be exploring Tomura's backstory in more detail. Maybe show his life before he became a villain, his family situation, and what led him to the path of destruction. Another could be a 'what if' scenario, like what if Tomura was redeemed and joined the heroes. He could use his knowledge of the villains to help the heroes take down other threats.
It could completely change the dynamics of the story. Maybe Deku would have a different perspective on villains and their motives.
Midoriya could be a great character to pair with Tsuyu. They both have a strong sense of justice and are very determined in their hero training. They could support each other in battles and share their experiences about growing as heroes.
Give Tomura a new power or ability that is unique to your fanfiction. This will set your story apart from others. For example, he could have the ability to control time for a short period.
One popular Tomura crossover fanfiction is the one where he crosses over with the Avengers universe. It creates really interesting scenarios as Tomura's power and motives clash with the superheroes of the Avengers. Another great one is his crossover with the Harry Potter world, seeing how his dark nature would interact with the magic there.
Well, obviously Mandalay herself is a main character. Then there could be her fellow heroes like Tiger. Students she mentors might also be main characters, like Kota who she has a connection to in the original story.
Uraraka could be an interesting side character too. Her friendly and upbeat personality could bring some light - heartedness to a story that might otherwise be quite serious when focused on Tsukauchi's detective work.
One great tomura x deku fanfic is 'The Unlikely Bond'. It delves into a world where Tomura and Deku have to team up against a common enemy and in the process, start to understand each other on a deeper level. The story has a great plot with well - written action scenes and character development.