Creative writing hasn't been one of the super-hyped use cases by OpenAI for the OpenAI API outside of AI Dungeon, surprisingly. For just random generation, the necessary curation can detract from the time-savings advantages. (as an aside, the API is also extremely expensive for long-form content to the point I'm not sure how the economics work for these startups even with charging monthly fees).
I'm more bullish on small bespoke models for a given use case, which is what I spend my time researching. (example: a ~1M parameter GPT-2 Magic: the Gathering card generator: https://colab.research.google.com/drive/1VOt090UzvltoBgMdUZm... )