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.
One possible scenario is that the OC is a healer. In the Baki world, especially when it comes to dealing with a wild and powerful being like Pickle, a healer OC could be very important. They might initially be scared of Pickle but then realize that Pickle is also in need of some form of care or understanding. So they start to interact with Pickle in a non - combative way, which changes the dynamic between them.
Well, in Baki fanfiction, an encounter between an Original Character (OC) and Pickle could be really interesting. Maybe the OC could be a fearless fighter who challenges Pickle. It could lead to a great battle where the OC tries to use their unique fighting style against Pickle's primitive and powerful moves.
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.
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.