This is somewhat inspired by  @3424896o6

I saw their KotLC/PJO crossover The Foster Children (great book from what's been posted so far, you should check it out) and it got me thinking, and I realized that you could technically create a crossover with almost any fandom by saying that the people there fostered Leo Valdez.

Example: With Land of Stories

Summary: For a couple of months when Alex and Connor Bailey were 8 or 9, their family was home for another kid named Leo Valdez, who was smart, funny, and the twins' best friend. However, a few years after they took him in, he ran away, and the twins were devastated, but they got over it, and they never thought of him or saw him again. Until now, that is. When Alex came over to see her brother in the Otherworld one day, she saw Leo again, and...