One way is by having them overcome their differences. Yeon may have a more brash personality while Bam is more reserved. By working through these differences, like when they have to make a joint decision in a life - or - death situation, their relationship can develop. Also, they could be put in a situation where they have to rely on each other's skills. If Yeon needs Bam's climbing ability and Bam needs Yeon's combat skills, this mutual dependence can enhance their relationship.
Common themes might include their friendship evolving in unique ways. For example, they could face challenges together that test their bond. Another theme could be their growth as characters in relation to each other. Yeon might influence Bam's view of the Tower, and Bam could inspire Yeon to be a better version of himself.
I'm not sure of specific Bam Bam Brown fanfic, but you can try looking on fanfiction platforms like Archive of Our Own. They often have a wide variety of fan - created stories for different topics and characters.
One interesting concept could be the combination of different power systems. For example, if the Tower of God characters cross over with characters from another fantasy world, how would their Shinsu-based powers interact with the magic or abilities from the other world. Another concept might be the clash of different cultures within the crossover. Characters from the Tower of God have their own set of values and social hierarchies, and when mixed with characters from a different universe, it can lead to unique story developments like alliances being formed or conflicts arising due to misunderstandings of each other's ways. Also, the exploration of new environments that are a blend of both worlds can be really engaging in such fanfictions.
Bam is definitely a popular choice. His unique power growth and his kind - hearted nature make him a great character to interact with characters from other universes. Khun is also popular. His intelligence and strategic mind can bring a different dynamic to crossovers. For example, if he were in a crossover with a detective - themed world, he could use his smarts to solve mysteries alongside the local characters. Another popular character would be Rak. His brash and strong - willed personality can lead to some really entertaining interactions, especially when paired with more reserved characters from other crossovers.