One idea could be a zetsu crossover with Naruto characters in a different timeline. For example, Zetsu interacting with a younger version of the Sannin. Another could be a crossover with characters from other anime like One Piece, where Zetsu might find himself on the Grand Line. Or a crossover with characters from a fantasy game world, and Zetsu has to adapt to the new magic system there.
Begin by choosing a crossover that excites you, say a zetsu crossover with the Star Wars universe. Consider Zetsu's personality and how it would mesh with the Star Wars characters. Is he more like a Sith or a Jedi in this new world? Develop a backstory for Zetsu in this universe. Maybe he was an experiment by the Sith or has a connection to the Force. Then, start writing from a point that makes sense, like Zetsu crash - landing on a strange planet in the Star Wars galaxy and having to figure out how to survive and what his purpose is there.
I'm not sure specifically as there could be many different interpretations in fanfiction. But it might involve Zetsu using his unique abilities to rescue Naruto from a dangerous situation, like a powerful enemy attack or a life - threatening trap.
One main theme is loyalty. Given their relationship in the Naruto series, fanfictions often explore how loyal they are to each other and their common goals.
I'm not sure specifically about 'Tobi x Zetsu' yaoi fanfiction. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net. They usually have a wide variety of fanfictions, and you might be able to find some related to this pairing there.
One common theme could be forbidden love. Since Zetsu has his own nature in the Naruto world and Sakura is a shinobi, their relationship might be seen as something that goes against the norms. Another theme could be the exploration of their different powers in relation to each other. For example, how Sakura's medical ninjutsu could interact with Zetsu's unique abilities.
One interesting sakura and zetsu fanfiction could be about them teaming up on a mission in a different dimension. They might face new and strange enemies, and through their combined efforts, they learn to understand each other better.
The theme of discovery is also prevalent. Kagome discovers new aspects of the Naruto world through Zetsu, and Zetsu discovers new emotions and ways of thinking because of Kagome. This mutual discovery creates a dynamic that drives many of the fanfictions in this pairing.
Well, one possibility could be that Zetsu imparts some unique and dark jutsu to Naruto. Maybe Naruto starts to learn stealth and subterfuge skills from Zetsu, which are different from what he learned at the ninja academy. This could lead to Naruto having an edge in certain battles where traditional ninja skills aren't enough.