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.
The Boku no Hero manga is finished. It reached its conclusion after a successful run, leaving fans with a complete narrative and memories of an exciting journey.
Well, 'boku no hero novel' is probably about the adventures in the world of heroes. It could have new storylines that didn't make it into the main anime or manga. It may focus on a particular character's growth, like Midoriya's journey in a different light, or introduce new characters and their unique quirks within the hero - filled universe.
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.
One popular idea could be creating an alternate origin story for a character. For example, giving Deku a different quirk acquisition experience that still leads to his growth as a hero.