[chimeracoder]

> It'd be interesting to have an opt-in for sharing your medical history with entities

That already exists. It's baked into HIPAA (if you're talking about medical history collected by Covered Entities).

All "medical" data collected by direct-to-consumer products that aren't providing clinical care (think 23AndMe, or third-party services that allow you to book appointments with doctors in a Yelp-style interface) are not subject to HIPAA, so they already can legally collect (and sell) that data without additional consent.