A great fanfiction idea could be a story where the Flame - using abilities in 'Katekyo Hitman Reborn' are incorporated into cooking in 'Food Wars'. For instance, the Dying Will Flame could be used to enhance the flavor or presentation of dishes. You might find such creative concepts in the fanfiction community.
When writing this kind of fanfiction, it's important to blend the two worlds seamlessly. You could start by creating a scenario where the two universes collide. For example, a mysterious portal might open and transport characters from one series to the other. Then, develop the relationships between the characters. The Hitman Reborn characters might be shocked by the intense cooking culture in Food Wars, and this can lead to humorous or exciting interactions.
In Food Wars, the story centers around young aspiring chefs at a prestigious culinary academy. They face various challenges and competitions to showcase their cooking skills and unique recipes. The series also explores their personal growth and friendships.
I recommend the following novels that were reborn in the Food Empire to you: The Legend of the God of Cookery, The Three Realms of the God of Cookery, A Taste of Food, Hi Bun's Dad, Reborn Babing Mi's Wife, There's a Black Shop in Online Games, Rice Worm Ensemble, Baiwei Garden, I Save the World with Food, Reborn Food Empire. These novels all had elements of rebirth and food. The protagonists established their own food empires through their own efforts and talents. I hope you will like one of them.