One idea could be that Mugetsu Ichigo somehow ends up at the High School DxD school and has to deal with the different factions there. He might use his unique powers to solve some of the school's problems.
The plot could involve Ichigo's unique abilities causing a stir in the Highschool DxD world. Since he has the power of Mugetsu, he could be a target for some groups who want to either use his power or get rid of him. And at the same time, he could befriend some of the characters from DxD and they could learn from each other's powers.
One idea could be that Ichigo is somehow transported to the High School DxD world and has to deal with the Devils and Angels there. He might use his Soul Reaper powers in unique ways to solve the problems in this new world.
A third idea could be a time - travel fanfiction. For example, a character from Highschool DxD travels back in time to the era of Bleach and meets Ichigo when he was still developing his powers. This could lead to some interesting interactions and changes in the timeline of both universes.
One idea could be that Mugetsu Ichigo finds himself in an alternate universe where he has to team up with his other - self from that universe to defeat a powerful new enemy. Maybe this enemy has the ability to nullify his Zanpakuto powers, forcing him to rely on new and creative combat strategies.
Maybe in the fanfic, Mugetsu Ichigo teams up with some unexpected characters from another series. For example, he could meet characters who have unique abilities that complement his own, and together they face a powerful common enemy that neither could defeat alone. This would lead to some interesting character interactions and power combinations.
If creating a new character in this fanfiction, let's say a student who is influenced by Mugetsu Ichigo. Their arc could start with being in awe of Ichigo's power. Then, through facing challenges together, they develop their own strength and identity, independent of just idolizing Ichigo.
He could fit as a powerful outsider. His power level is extremely high with Mugetsu, so he'd be a force to be reckoned with. Maybe he'd be seen as a wildcard by the different groups in DxD.
In some fanfics, a great plot twist is when Ichigo's Hollow powers are misinterpreted by the DxD characters as some sort of devilish power. This leads to a lot of misunderstandings and conflict at first. Another interesting plot twist can be when Ichigo somehow ends up being related to an important figure in the DxD world through some convoluted backstory.
One possible plot could be that Ichigo somehow gets transported to the world of High School DxD. He might initially be seen as a threat by the devils and angels there due to his unknown powers. But as they learn about his motives of protecting people, they start to work with him. Ichigo could also teach the characters in High School DxD about his unique fighting style and the concept of Hollow and Soul Reaper powers, leading to new and exciting battles against common enemies.
A more complex plot could be centered around a prophecy that foretells the destruction of the Highschool DxD world. The characters then have to embark on a journey to find the source of this prophecy and try to prevent it. Along the way, they discover secrets about their own powers and the true nature of the different factions, and have to make difficult choices that could either save or doom their world.