Crossovers

We see them often as the type of fanfictions written by writers(of course xD). If you don't know what a crossover fanfic is, a crossover is simply a fanfiction that combines elements, characters, etc from 2 or more different franchises.

So what exactly makes a good crossover? And what should a crossover fanfic NEVER EVER contain? Do some crossovers work? If they don't, why? Let this be a place to discuss crossovers~!

I haven't ever tried a crossover in the sense of making a fic with elements/characters from multiple fandoms. I have bene in an (unsuccessful) writing project with multiple writers though using characters from their own fic, but that fell through due to multiple reasons.

I would say that only some combinations of crossovers would work (unless the author was very good at say the crack style of fic for some). Furthermore what would need to be considered is how/why the two universes, so to speak are mixed together, and how characters from one side side would react to the other set of characters/setting/whatnot. How they mesh together would be certainly a main point to consider though imo.

On that note, I wonder what people would make of some of the potential combinations offer by this site...