To me, I've thought of writing IY/DBZ crossovers, mainly because of the storylines I could come up with in regards to a Kagome/Vegeta pairing. I like to compare different fandoms and see which characters are either alike, or might make good pairings. I never watched YYH, so I wouldn't be able to comment.
I've thought up ways to integrate Vegeta into the IY universe. For example, he's a half demon, transforming into a giant ape when he looks at the moon. He's very powerful, but evil. But Kagome is able to soften his heart, sort of like what happened in the DBZ series. Stuff like that.
The latest crossover project I'm taking into consideration is a Slayers/Inuyasha crossover. You have Zelgoddes, who is a half demon with an attitude problem, like Inuyasha, so I was planning to have the two of them go Yaoi on each other, which explains why Inuyasha never went with Kagome.
. Kagome makes friends with the nice priestess Sylphiel, Shippou and Amelia make a good pairing. The pervert Miroku and Gourry make another good pairing. So does Lina Inverss and Sango for beating up on their boys and slaying things in general.
So yeah, lots of ideas, but basically fandom comparisons is my main reason at least for doing crossovers.