One common theme is power and ambition. Both Madara and Obito are characters with great power and strong ambitions in the Naruto series, and fanfics often explore how their relationship is influenced by these aspects.
The theme of redemption also appears. Given some of the actions of Obito and Madara in the Naruto series, fanfics sometimes explore if they can find redemption in their relationship with each other. They might try to make up for their past mistakes through their connection in the fanfic.
A third option is 'The Alliance of Obito and Madara'. In this one, the author focuses on their combined power and how they could have ruled the ninja world together. It's full of action scenes and also has some deep emotional moments between the two characters.
In such a fanfiction, the story could take a very dark turn. Naruto's ideals of friendship and peace might be severely tested. He could be forced to question his own beliefs as Obito and Madara have very different outlooks. Maybe Naruto would try to change their minds, but it would be extremely difficult as they are both very set in their ways of achieving a new world order through more extreme means.
Fanfiction.net is also a good place to look for madara x obito fanfic. There are many different stories written by fans from all over the world. However, the quality may vary, so you might need to read the summaries and reviews before choosing one.
A fanfiction story could start with Naruto being in a desperate situation where he feels that the normal path to peace is not working. So, he decides to team up with Obito and Madara. At first, they seem to be working towards a common goal. But then, Naruto realizes that their means are too extreme. For example, he discovers that the Infinite Tsukuyomi they plan would take away people's free will. He then has to find a way to stop them while still maintaining some sort of alliance. This could lead to a complex web of relationships and power struggles within the group.
Well, it could be about Kakashi finally finding Obito after a long search. Maybe Obito has changed a great deal, and Kakashi has to come to terms with this new version of his old friend. They might have a battle first due to their different paths, but then they start to talk and remember their past together, like their time as teammates in Team Minato.
One common theme is redemption. Since Obito has done a lot of bad things, in many fanfictions Sakura becomes a catalyst for his redemption. For example, she might show him the good in the world again.
One possibility in the 'naruto meets obito fanfiction' is that Naruto and Obito meet in a different dimension. Naruto is shocked at first to see Obito in this new setting. Obito, on the other hand, is still full of anger and resentment. But Naruto, through his unwavering belief in the goodness of people, starts to break down Obito's defenses bit by bit. They might exchange stories about their lives, with Naruto sharing his dreams of peace and Obito revealing his past traumas. Eventually, this could lead to a change in Obito's character.