Many users rely on proxy/VPN tools without knowing if they're truly secure. A service could systematically test these tools for common vulnerabilities like certificate bypass issues.
Build automated test suites that check for known vulnerabilities, plus manual code review for critical tools. Offer public reports and private audits for vendors.
Charge tool developers for certified audits, and offer basic scans free to build reputation.
MVP is a simple scanner for the most common vulnerabilities, with manual reports.
Biggest risk is keeping up with new attack vectors in this fast-moving space.