Why does it say "If you could make an animorphs crossover..." by the way? I mean if we wanted to do one we would do one...
As Mr. 281 above said, crossovers are no easy task. It's one thing to say "I think Animorphs and Blue's Clues would fit well together" and quite another thing to actually work out the differences between universes, character interactions, and whatnot. It's too easy to make a bad crossover because many novice writers simply throw in characters from their favorite fiction series and it all looks out of place. I know many people who look down on the idea of crossovers in general because they have such a negative reputation.
Case in point: I'm working right now on a Pokemon fan fiction series that eventually incorporates elements from other fiction, in effect making it a crossover (see
here for a mind dump. TL;DR version: I spend too much time on this). This general concept has been in the works since early 2004 (perhaps late 2003). It is still being refined.
It's bad enough taking on ordinary fanfiction, as it's difficult enough to portray a single universe correctly. With crossovers, the problem is not only doubled but squared, since you have to worry not only about the accuracy of the individual universes but also about the interactions between universes.
Or you can avoid all of that and label it an alternate universe fic. That sometimes works.
