This is why there's an entire section on fanfiction dot net just dedicated toward crossovers, and why I write them. I also enjoy role playing within crossovers too. There's more freedom.
I especially like the little nuggets of crossovers. Like in Negima!?, Nodika got her book and started making a wish... turned out to be nearly identical to the wish Keiichi uttered to Belldandy in Ah! My Goddess.