Hana-hacking
Katsuki will be hacking up a lot of flowers seeing as he has a tendency to not open up. Potential topics may include his regrets over his past bullying (this one can be triggering so if you want to do something let me know so I can include all the warnings), being kidnapped and a lot of people getting hurt in the process of rescuing him, considering himself weak, inadequate etc and his insecurities that his classmates will get so ahead of him that he won't be able to catch up, and the fact that he wants to bring his teacher Midnight here somehow so that her life will be saved.

Uproot of the Spirit
I'm tempted to have Katsuki be influenced by two gods:

Ainea: Katsuki will try to give his grenadier bracers he wears and any other grenades and bombs he's made away.

Rawna: Who wants to be comforted by someone who really sucks at comforting? He'll try to sooth others, even when nothing's wrong and offer to kill their problems.

Cause I'd Get A Thousand Hugs
Katsuki is making an effort to hug people more, so if anyone wants to do this prompt with him I'd love to do it!