A crossover with 'One Punch Man' would also be cool. Saitama's nonchalant power compared to Baki's intense training and determination could create an interesting dynamic. Maybe Baki tries to figure out Saitama's secret to his strength.
The best way to know where to start is to look for discussions on manga-focused websites. Fans often share their experiences and advice on continuing after the anime. Another option is to check social media groups dedicated to Baki Hanma to get suggestions from fellow fans.
Focus on character development. For example, if you cross Baki with a character from another series, show how they influence each other's growth. Let's say it's a crossover with 'Naruto'. Baki could learn about the power of friendship from Naruto and Naruto could learn about Baki's extreme physical training methods.
One common theme is friendship. Despite coming from different backgrounds, Naruto and Hanma often form a bond in these fanfictions. Another theme could be power - struggle, as they compare and combine their different powers.