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.
Canonically, Fang is just Hagrid's pet that Hermione and the others encounter now and then. But in a lemon fanfic, their relationship could be completely transformed. Hermione might start to communicate with Fang on a different level, almost as if they can understand each other's thoughts. Their interaction could involve shared secrets or a mutual understanding of a situation that others don't know about. This would be a big departure from the simple, more surface - level interactions in the books.
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.
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.
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.
In a fanfiction where Naruto is a rurouni meeting Hinata, their interaction would have a different dynamic. Canon Naruto was always in a rush, trying to achieve his goals. As a rurouni, he's more laid - back. So when he meets Hinata, he might actually take the time to have a long, deep conversation with her. They could talk about things like their dreams for the future of Konoha, or their personal insecurities. This would be very different from the quick exchanges they had in the original story.
Their interaction could be quite interesting. Goku might be curious about Android 21's abilities, always eager to test his own strength against hers. Android 21 could be initially guarded but gradually open up to Goku's friendly and straightforward nature.
In the original story, Luffy is male and his interactions with Mihawk are based on male - male dynamics. In a fanfic with Female Luffy, there could be an added layer of romantic tension or a different kind of respect. For example, Mihawk might be more protective or show a different side of chivalry towards Female Luffy.
In the original Harry Potter canon, the Master of Death is more of a theoretical concept. But in fanfiction, it can be much more fleshed out. For instance, in fanfic, Harry might actually be able to control death on a large scale, like preventing natural disasters that cause death in the wizarding world, which is not something in the books.
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.
It probably wouldn't fit directly into the main canon as it's a rather light - hearted and comical concept. But it could exist as a side - story or an 'alternate universe' type of fanfic. It can show the characters in a more relaxed and fun - loving situation that we don't often see in the main series.