What features are AI Subject Line Generators missing?

We launched Prompt Hunt 2 months back, and many prompts were for generating subject lines. It seemed to be a popular use case, so I researched a bit and realized that some of the online tools could be better. Wanted to know all of your opinion about this. What features an AI tool for the subject line should have? (We're planning on launching one soon) Also, check prompt hunt if you haven't yet -> https://www.mailmodo.com/ai-prom...