In a crossover with 'Naruto', Naruto himself would be a great addition. His positive and determined attitude could contrast with Tomoe's more reserved nature. And his powerful ninja skills could come in handy when facing some of the demons in the Kamisama Kiss world. Sakura could also be there, and she might bond with Nanami over their strong wills.
If it was a crossover with 'Fullmetal Alchemist', Edward Elric would be an interesting character. His knowledge of alchemy and his search for the Philosopher's Stone could somehow tie into the magic in the Kamisama Kiss world. Alphonse could also be included, and the gentle giant could form an unlikely friendship with some of the yokai in Kamisama Kiss.
For a crossover with 'Sailor Moon', Sailor Moon herself would be a natural choice. Her ability to fight evil and her kind heart would make her fit right in. She could team up with Nanami and Tomoe to fight against a new threat that combines the dark forces from both the Sailor Moon and Kamisama Kiss universes. The other Sailor Scouts could also be involved, each bringing their own unique powers.
The best chapter pointer here is Chapter 13, "Killing the Chicken to Scare the Monkey". It opens with Sensing that something was off with Jiang Yan, Jiang Yan dismissed the... and closes on How could this happen..." Lin Manru didn't want to hear any more of her....
Chapter 778, "The Gift of Being a Director", is the clearest early answer because it turns Shiguang's situation into a concrete plot event. The chapter gives readers a real answer by opening on Shiguang, I really misjudged you, you're actually this kind of...
Chapter 644, "Has Shang Shaojing Actually Gone Bankrupt?!", is the clearest early answer because it turns Actually Gone Bankrupt's situation into a concrete plot event. Early on, Su Yinwan was already on her way home when she saw the trending....
Chapter 630, "The Secret Manual Passed Down to Sons, Not Daughters", is the clearest early answer because it pushes Xiao Wu into a new phase or setting. The chapter turns that into something concrete by moving from Following the command, Xiao Wu, clutching the anesthetic gun,...
Chapter 819, "Invitation", is the clearest early answer because it turns Jiang Shixu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with What Yu Li said is something these prominent families often do.
Chapter 552, "Branches, 20 Points Each, No More", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event. It opens with Jiang Cheng smelled the scent and crawled out of the shed, but before...
Chapter 861, "Add the Link!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. It opens with At exactly ten in the morning, Jiang Li started her live stream. and closes on Changqing curiously leaned over for a look, and his ears instantly....
Look to Chapter 11 for the clearest answer. It begins with After a meeting, Shang Shaojing pinched the bridge of his nose, tired, and..., keeps building through How is it going with the Jiang Family?" The assistant looked troubled and..., and lands on Miss Qin, please accompany us to the police.
If you want the clearest chapter reference, go to Chapter 10. Early on, Qiao's shocked face struggle to recover, Lin Manru also tasted for the...; later, the chapter leans on Jiang Li nodded with a smile, seemingly approving of Lin Manru's words and....
Chapter 864, "What if he just wants to take revenge on you?", is the clearest early answer because it turns Jiang Meng's situation into a concrete plot event. Early on, I did see the robbery star in your Life Plate..