You can try fanfiction.net. It has a vast collection of fanfics from various fandoms, and there's a good chance you'll find some 'harry x madara fanfic' there.
One great 'harry x madara fanfic' could be 'The Unlikely Alliance'. It explores how Harry's magic and Madara's powerful jutsu could interact in a unique way. They start off as enemies but gradually come to understand each other's motives.
I really like 'Eternal Connection'. In this fanfic, Madara and Itachi are brought together in a time - travel scenario. It's a really interesting concept as they both have to deal with the knowledge of the future and past while also developing their relationship. It's full of action and some really touching moments.
Power is a common theme. Both Madara and Sasuke are extremely powerful characters, so fanfics often explore how their powers interact or complement each other. Another theme is redemption. Madara has a complex past, and Sasuke has his own journey of redemption in the original story, so this can be explored in their relationship in fanfics.
Another theme could be the power dynamic. Madara is a very powerful character, and Hinata has her own unique abilities. Fanfics might play with how their powers influence their relationship, like how they protect each other or train together.
The theme of unrequited love is also quite popular. It's often depicted that one of them has feelings that the other is initially unaware of. And there's the theme of power balance. How their powers affect their relationship, whether it brings them closer or drives them apart.
Love against all odds is also a prevalent theme. Given their different backgrounds and the circumstances in the Naruto universe, their love is often shown as something that overcomes great barriers. It can be things like clan differences or past enmities that they have to surmount to be together.
One interesting Sasuke x Hinata Madara fanfic is 'The Unlikely Bond'. It explores a unique scenario where Sasuke, Hinata and Madara are brought together in a complex plot. The characters' personalities are well - written and the story has a good flow.
In 'madara x mito fanfic', a common theme is the cultural differences between their clans and how they navigate through them in their relationship. Their individual goals and how they either support or conflict with each other's are also explored. And sometimes, the theme of redemption can be present, especially if one of them has made mistakes in the past and the other helps them to overcome it in the context of their relationship.
A third common theme is the changing of destinies. Naruto is all about defying fate, and when paired with Madara, who has his own ideas about the world's destiny, fanfics often explore how they can change or shape the future of the ninja world together.