Maybe it's because your character fits well into the alternate storylines or has unique traits that make for an interesting crossover.
One main theme is adaptation. Naruto has to adapt to the rules and the magic - based world of Re:Zero. And vice versa, the characters in Re:Zero might have to understand Naruto's ninja world concepts.
Another possibility is a crossover with 'My Hero Academia'. Subaru could be a student at U.A. High School. His Return by Death power would be like a quirk. He might struggle at first to control it and fit in, but eventually use it to save his classmates from villains, in a way that surprises everyone with his growth and resourcefulness.
One possible plot could be that Subaru from 'Re:Zero' somehow gets transported to the world of 'Young Justice'. He might initially be confused but then use his 'Return by Death' ability to his advantage. For example, he could help the Young Justice team in battles by being able to predict enemy moves after dying a few times. His unique perspective on life and death could also influence the team's moral decisions.
First, decide which other universe you want to cross Re Zero with. Then, research both universes thoroughly. Know the characters, the rules, and the settings. For example, if it's a crossover with 'Naruto', understand how chakra works and the different ninja clans. Next, come up with a central plot that ties the two universes together. It could be something like a common enemy or a shared goal.
A Konosuba - Re:Zero light novel crossover might explore the different magic systems. In Konosuba, magic is often misused or has unexpected results, while in Re:Zero, magic has a more serious and often dangerous undertone. The crossover could play with how the characters from each series view and interact with the other's magic. Also, the settings are quite different. Konosuba has a more light - hearted and comical fantasy world, while Re:Zero's world is darker and full of political intrigue. Combining these two settings could create a really unique story.
There's also 'Re:Zero with Naruto's Powers'. This fanfiction often focuses on how the characters in Re:Zero react to Naruto - style powers. For example, Subaru might be both amazed and a bit envious of Naruto's chakra control and his powerful jutsu. It can create some really unique interactions between the two very different universes.
Another plot could involve the Pleiades from Overlord. They end up in Re:Zero and are mistaken for some sort of magical spirits. Subaru tries to enlist their help in his quest to save Emilia. The Pleiades, with their combat skills and unique abilities, could change the power dynamics in the Re:Zero world and also introduce new technologies and magic concepts from Overlord.
The interaction between Ainz and Roswaal would be fascinating. Ainz is used to being in a position of power and having complete control over his domain in Overlord. Roswaal, on the other hand, is a complex character in Re:Zero with his own motives and schemes. Their encounter could be a battle of wits as they try to outmaneuver each other, or they might find common ground in their pursuit of power and knowledge.
One possible plot could be that Subaru finds himself in the world of My Hero Academia. He might initially be confused by the existence of quirks. Since he has the ability to return by death, he could use this to his advantage in battles against villains. For example, he could keep trying different strategies to defeat a powerful villain like All For One, learning from each death and getting closer to victory each time. He could also form friendships with the students at U.A., sharing his own experiences of struggle and perseverance which would inspire them in return.
Definitely Subaru from 'Re:Zero'. His 'Return by Death' ability makes him a really unique character. In a crossover, seeing how he interacts with the super - powered heroes of 'Young Justice' would be fascinating. Also, Robin from 'Young Justice' could be interesting. His detective skills and leadership could contrast well with Subaru's more impulsive and desperate style of dealing with situations.