Developers waste time researching which image format to use for different scenarios. Build an interactive tool that asks about their specific needs (transparency, animation, compression needs) and recommends formats with data-backed explanations.
The tool could show real compression comparisons and browser support charts. Monetize through affiliate links to image optimization services and sponsored placements from CDNs.
Start with a simple static site with comparison tables, then add the interactive advisor. The risk is browser support changing rapidly and making recommendations obsolete.