In this kind of fanfiction, Naruto being heartbroken by Rias could be due to a variety of reasons. Maybe Rias betrays Naruto in some way, like choosing someone else over him in a crucial situation. Or perhaps there are misunderstandings between them that lead to Naruto's heartbreak. It could also be that Rias has to follow some rules in her world that go against Naruto's expectations, causing their relationship to shatter.
One aspect could be that the fanfiction shows Naruto's heartbreak in a very visual and emotional way. For example, he might see his parents in a dream - like state and then they disappear, leaving him feeling more alone than ever. It's a way to dig deeper into his character's vulnerability, which was somewhat masked in the original series by his determination.
Betrayal is another big theme. When someone Naruto trusts turns against him, it breaks his heart. It often leads to him feeling isolated and having to re - evaluate his relationships. For example, if Sasuke were to betray him in a more extreme way in these fanfics. And it's not just about the act of betrayal but also the emotional aftermath that Naruto has to deal with. He has to face his own feelings of hurt, anger, and confusion. Sometimes, he might even start to doubt his own judgment in trusting people.
One great Naruto heartbroken crossover fanfiction is 'Naruto in the World of Bleach: A Broken Heart's Journey'. In this fic, Naruto is transported to the Bleach universe and his experiences there, along with missing his friends from his own world, lead to a heartbroken state. It has great character development and an interesting plot that really delves into Naruto's emotions.
One possible reason could be the loss of a dear friend. Naruto values his friends deeply, and if he were to lose one, it would break his heart. For example, if Sasuke were to die or turn against him completely in a way that Naruto couldn't fix, it would be a huge blow.
One possible reason could be a failed relationship. In the DxD fanfiction world, Naruto might fall in love with a character, but due to differences in their goals or the interference of others, the relationship fails. For example, if he falls for a devil girl who has to follow the orders of her clan which oppose her being with Naruto. Another reason could be the loss of a dear friend. Maybe one of his newfound friends in the DxD universe betrays him or gets killed, leaving him heartbroken.
One great 'naruto heartbroken fanfiction' is 'Broken Promises'. It really delves into the emotional turmoil of the characters. The author does an amazing job of showing how Naruto's world can be shattered by unfulfilled expectations and lost relationships.
One possible plot could be that Naruto discovers that his friends have been keeping secrets from him that make him feel betrayed. For example, they might have made a deal with the village elders that goes against Naruto's values. This would break his heart as he thought they were all in it together. Another idea is that the villagers, who he has tried so hard to protect and earn the respect of, suddenly turn on him due to false rumors spread about him.
Well, the theme of powerlessness often comes up. Despite being godlike, he's powerless to stop the source of his heartbreak. There might also be the theme of regret. He could regret not doing something different that could have prevented the situation. And the theme of change is common too. His heartbreak would change him from the cheerful Naruto to a more somber and withdrawn version.
The contrast between the two universes. In a Naruto and Attack on Titan crossover, Naruto's heartbreak might be amplified by the brutal and different world of Titans. The new challenges and how he copes with his broken heart in such a different environment make for a great story. Also, exploring his powers in a new context while being heartbroken adds depth.