I nominated Original Work and used non-canonical characters, pairings, and tags. For proper science I'd have to try non-canonicals in each field in turn.

EDIT: Non-canonical characters gets "These character tags in your request are not canonical and cannot be used in this challenge". Non-canonical relationships gets "These relationship tags in your request are not canonical and cannot be used in this challenge". Non-canonical additional tags gets the "Edit Prompt" screen refreshed with no error message but no changes saved.