Well, there's a "multifandom" tag which might be applicable for things that crossover numerous fandoms or are applicable to numerous fandoms. Also, I'd guess that as the archive begins to host more than text there will be growing use of format tags so that a post of filk lyrics would be different from a filk recording. But one could simply label it as Filk. And wouldn't kink_bingo be the Collection? I'm not that familiar with it but I gathered it's a sort of ongoing challenge community?