A possible plot could be that Android 18 and Goku are thrown into an alternate dimension where they have to rely on each other to survive. In this dimension, they face challenges that test their friendship and fighting skills. Android 18 might learn some of Goku's signature moves and Goku could learn more about the technology within Android 18.
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 plot idea could be Android 17 in a post - apocalyptic world, using his powers to protect the few remaining humans from other threats. Another could be Android 17 getting transported to an alternate universe where he has to team up with his evil self to defeat a greater enemy. Maybe he could also have a story where he tries to integrate into human society and discovers emotions he didn't know he had.
The plot might involve Naruto trying to prank Android 17 by challenging him to a fake competition. Naruto could pretend to be really bad at it at first, making Android 17 overconfident, and then suddenly show his real skills and win in a very unexpected way, leaving Android 17 shocked.
Maybe in the fanfic, Gohan is the first one to notice Android 18's different side. He begins to befriend her, and as they interact more, Android 18 falls in love. Their relationship then faces challenges from the Z - fighters who are initially suspicious of Android 18's motives.
The plot could be about a power struggle. Cell, always seeking more power, might see Android 18 as a key to achieving ultimate strength. Android 18, on the other hand, would resist his attempts. This back - and - forth struggle could form the core of the fanfic.