Friendship is a very common theme. Given their relationships in the original story (assuming it's One Piece - related), fanfictions often explore the deep bond between Marco, Ace, and Thatch. They might show how they support each other through various challenges.
One popular place to find marco x ace x thatch fanfiction is Archive of Our Own (AO3). Just go there and search for the relevant tags. You'll likely come across a variety of stories with different themes and plotlines.
Yes. The conflicts between Sanji and Luffy in the manga often stem from complex circumstances and character developments that drive the narrative forward.
The popularity also stems from the fact that there are so many unexplored aspects of their relationship in the official story. Fanfiction allows writers to fill in those gaps and create stories that satisfy the fans' curiosity. For example, their early days together or how they would interact in different settings.
There might be. However, it's important to note that this kind of fanfiction is more of a creative and often not part of the official storyline. Some fans really enjoy exploring this kind of relationship between Zoro and Sanji in their own fictional interpretations. They might add interesting plot twists and character developments to make the kiss scenario seem more believable within the world they create in the fanfiction.
Often, they involve a lot of emotional build - up. Since Zoro and Sanji have a complex relationship in the original story, in fanfiction, the kiss might be a result of resolving their differences or a sudden realization of deeper feelings.
I think it might have been a character introduced to add depth to Sanji's backstory and motivations. But without more context, it's hard to say for sure.