A fanfiction could work by creating a scenario where the goals of characters from both universes align. For instance, if there is a greater evil that threatens both the ninja world of Naruto and the supernatural world of High School DxD, the characters could team up. This would lead to interesting interactions as they learn about each other's cultures, powers, and ways of fighting. For example, Naruto's team -work based combat style could be a new concept for the more individualistic characters in High School DxD. And vice versa, the High School DxD characters' use of magic and different forms could be fascinating for Naruto characters.
It can work by finding common elements between the two. Both have elements of power systems, like chakra in Naruto and Sacred Gears in High School DxD. So, a story could revolve around comparing or combining these power systems.
One way it can work is through a portal - type situation. A portal opens between the two worlds, and characters from each side start exploring the other world. Naruto characters might be shocked by the more modern and sometimes raunchy setting of High School DxD, while the High School DxD characters could be in awe of the more traditional and martial - arts - based world of Naruto. This would lead to a lot of humorous and also tense situations as they try to coexist and deal with any threats that come their way.