Perhaps an interaction where Android 18 saves Master Roshi from a danger. This turns their relationship on its head as Roshi now owes her one. He then tries to repay her in his own way, like offering to train her for free or sharing some of his secret techniques. This would create a dynamic where they are no longer just acquaintances but have a more complex relationship.
One interesting interaction could be when Android 18 challenges Master Roshi to a sparring match. Roshi, despite his age, shows some unexpected moves which surprises Android 18. This could lead to her respecting him more.
The fanfic could also revolve around a mission. For example, there's a new threat in the Dragon Ball universe and Android 18 and Master Roshi team up to face it. Along the way, they have to overcome their differences. Android 18's cold - blooded nature (initially) clashes with Roshi's more laid - back and traditional approach, but eventually they find a way to work together effectively.
A third storyline could be that Android 18 is sent by the Red Ribbon Army remnants to spy on Master Roshi. But as she spends time with him and his students, she starts to question her loyalties. Roshi, sensing something is off, tries to reach out to her. Eventually, she defects from the Red Ribbon Army and becomes a true defender of Earth, learning martial arts from Roshi along the way.
Well, in canon, they don't really interact much. In a fanfic, their interaction could be full of surprises. For instance, Android 18 might be much more brash and direct towards Roshi compared to how she behaves with other characters. Since she's an android, she may not have the same respect for Roshi's age - old traditions right away.
To write their character interactions well, first understand their personalities. Bulma is intelligent, sassy, and independent. Roshi is a wise but also a bit of a pervy old martial artist. Have them play off each other's traits. For example, Bulma could roll her eyes at Roshi's old - fashioned ideas while Roshi could be impressed by Bulma's quick thinking. Use humor in their exchanges, like Roshi making a clumsy attempt at using one of Bulma's gadgets and Bulma teasing him about it.
Android 18 in a fanfic with Roshi could be a source of conflict or resolution. She might initially be at odds with Roshi due to her android nature and his more traditional ways. But as the story progresses, they could find common ground. Maybe she has a mission that Roshi unknowingly gets involved in, and together they have to face various challenges, which in turn builds an unexpected friendship or partnership.
Roshi could be depicted as a comical yet endearing character in relation to Android 18. He might be constantly amazed by her android abilities, like her super strength or energy blasts. At the same time, he tries to impress her with his own martial arts skills, which might seem a bit primitive compared to her technology - based powers, but still hold a certain charm and wisdom.
One interesting story could be Master Roshi imparting his unique wisdom, which is different from the normal Naruto teachings, to a young Naruto. Maybe he shows Naruto some unorthodox ways of chakra control.
One plot idea could be that Master Roshi discovers a new form of martial arts with the help of Lemon. Maybe Lemon has some unique abilities that she shares with Roshi, and together they develop a new style that combines her special skills with his vast experience. For example, if Lemon can communicate with animals in a special way, Roshi could use this to train in a more nature - oriented martial art.
There could be a fanfiction where Master Roshi discovers a new form of ki energy. He then tries to teach this to Goku and the others. In terms of the 'lemon' which is often associated with inappropriate content, we should shift our attention to the potential of the story in terms of exploring the Dragon Ball Z lore more deeply. For instance, how this new ki energy could change the power dynamics between the characters and lead to new battles and alliances.
One plot could be that Roshi enters a cooking competition, and Bulma decides to help him out using her scientific approach to cooking. They combine Roshi's traditional recipes with Bulma's high - tech cooking gadgets and create amazing new dishes. Along the way, they face off against other competitors who are jealous of their unique approach. Another idea is that a strange alien race kidnaps Roshi, believing he has a powerful energy within him. Bulma uses her intelligence and resources to track him down and rescue him. During the rescue mission, they learn more about the alien race's motives and end up befriending some of the aliens who were against the kidnapping in the first place.