Well, a common theme is the alteration of relationships. With the Evil Rinnegan, Naruto's relationships with his friends and allies could change drastically. For example, some friends might be scared of him or turn against him. And in terms of the story's setting, it often involves a darker, more dangerous world. Since Naruto has this evil power, the balance of power in the Naruto universe is disrupted. Enemies might be more aggressive or new, more powerful enemies could emerge, drawn to the power of the Evil Rinnegan. It also often has themes of mystery, as the origin and true capabilities of the Evil Rinnegan in these fanfictions are usually something to be uncovered.
The theme of isolation is often present. Naruto, having the Evil Rinnegan, might find himself isolated from those he once cared about. His new power could be seen as too dangerous or his actions too extreme. Another theme is the exploration of different jutsu combinations. The Evil Rinnegan might give Naruto access to new and terrifying jutsu, and fanfictions might explore how he uses these in battles. And sometimes, there's a theme of a hidden agenda. Maybe there's a greater force behind Naruto getting the Evil Rinnegan, and the story unfolds as he discovers this hidden agenda and decides whether to go along with it or fight against it.
Chapter 594, "Raising Soldiers for a Thousand Days, Using Them for a Moment", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event.
Chapter 332, "Start the Live Stream!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event.
Chapter 587, "Has He Also Become a Part of the Play?", is the clearest early answer because it turns Zhao Lang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Zhao Lang said "thank you," but his eyes reluctantly glanced at Jiang...
Chapter 519, "I'm Here to Participate in the Program", is the clearest early answer because it turns Jin Siyue's situation into a concrete plot event.
Chapter 285, "Ksitigarbha Bodhisattva", is the clearest early answer because it turns Primordial Forest's situation into a concrete plot event.
Chapter 505, ""Postpartum Care for Sows", is the clearest early answer because it turns Shang Zhenhe's situation into a concrete plot event. It opens with Zhao Lang laughed, but his eyes became considerably clearer, "Is...
Chapter 498, "How can someone be so ugly?!", is the clearest early answer because it turns Shang Shuyue's situation into a concrete plot event. The chapter gives readers a real answer by opening on .....If you don't love me, give my heart back, you traded love for my...
Chapter 619, "The Feast of Fish CP Is Indeed Real!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The storm grew fiercer until, at last, even the live broadcast had to...
Chapter 420, "Who Says the Attacker Can't Attack the Attacker?", is the clearest early answer because the buildup finally cashes out in a real clash. The chapter gives readers a real answer by opening on After choosing to defend, Guan Xiong quickly made a decision.