Studies of the reasons fans transform texts for the most part leave unanswered the question of why particular kinds of texts are transformed by fans more frequently than others. All texts are technically available for fan transformation. However, not all texts are equally transformed. It is an objective fact that some TV shows attract more transformative fan engagement than others. In this article, I argue that formal properties of texts make them more likely to be subject to transformative fan works, i.e. more transformable. Shows that (1) posit a plastic universe that more easily accommodates ‘play’ than realism, (2) involve stories that follow some of the narrative conventions of melodrama and (3) display main characters with ‘ship’ – especially homoerotic ‘slash’ – potential have transformable canons. To prove this thesis, I examine two of the shows – and their respective fanfiction – that have the most fanfiction on FanFiction.net and ArchiveOfOurOwn.org: Supernatural and Sherlock.