One common theme is growth. Both characters are often shown to grow as individuals and in their relationship with each other. For example, Mori Jin might learn more about patience from Mira, and Mira could learn about taking risks from Mori Jin.
One idea could be to cross Mori Jin with a character from a different anime like Naruto. Mori Jin could end up in the Naruto world and have to deal with different jutsus and the ninja way. He might even teach some of the ninjas his own martial arts skills.