One possible plot for a 'kisame kidnaps sakura fanfic' could be that Kisame mistakes Sakura for someone else who has wronged his organization. He quickly grabs her and takes her away. Sakura, initially terrified, soon realizes the misunderstanding. She then tries to convince Kisame of his error while also looking for opportunities to free herself. Meanwhile, her friends from the village start a search and rescue mission, leading to a series of intense confrontations between the rescuers and Kisame's allies.
Sakura would probably be initially very angry. She has strong feelings for Sasuke, but kidnapping is a very extreme act. So she would likely fight back, using her medical ninjutsu skills and combat abilities to try and escape.
It could be that Itachi is being controlled by some dark force or an enemy. He doesn't really want to harm Sakura but has no choice. The fanfic might then focus on Sakura trying to figure out how to break the control on Itachi while also trying to escape from the situation. She could use her medical ninjutsu skills to her advantage in some way.
One possible plot could be that Sasuke has some sort of misunderstanding. Maybe he believes Sakura has some information about a threat to Konoha that she's not sharing, so he kidnaps her to force her to talk. Then Sakura tries to convince him that he's wrong while also trying to understand why Sasuke has become so extreme.
Sakura is smart, so she might try to outwit Madara by pretending to be compliant at first. She could start a conversation with him, trying to find out his motives and weaknesses. Then, when the time is right, she could use her chakra control skills to break free from any bindings and make a run for it. Maybe she would also try to send a hidden signal to her teammates if she gets the chance.
In such a fanfic, perhaps Itachi kidnaps Sakura for some strategic reason related to the shinobi world. Maybe he thinks Sakura has some knowledge or ability that could be useful to his goals. For example, her medical skills could be crucial for treating someone in his group.
Yes, there might be such fanfictions in the vast world of fan-created content. Some fans like to create unique family relationships between different characters from various anime, and this could be one of those interesting concepts.
Yes, there are some out there. You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'kisame and sakura' and you'll find various stories that explore their relationship in different ways.