What would actually be worth paying for in a prompt optimizer? (Asking before I build a Pro tier)

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PromptEngineering

I built as a side project. Takes a vague prompt, rewrites it into something more structured. Free, no signup. It's processed 71,000+ optimizations so far. It's at the point where I'd like to make it sustainable, so I'm thinking about a Pro tier. Before I build anything, I want to ask the people who'd actually use a tool like this: 1. What features would be worth paying for? Realistic price range $5-15/month. Some things I've been considering: - Saved prompt library with tags and search (current