This is quite good. Maybe not the direction you're thinking of taking this, but it would be nice to have some optional social features; e.g. community writing prompts, commenting, etc. I used to use a site that would give you a 5 minute writing prompt and then allowed other users to read and comment on the daily entries. It was really fun and encouraging! I'd love to have an "HN Daily Writers" group to practice with.
Incidentally, it would also make the subscription a much more compelling value proposition.
Incidentally, it would also make the subscription a much more compelling value proposition.