We’re in the process of migrating to the Add On Index. I think its download button links directly to the artifact on bintray, so not sure where you’d collect an email address.
Maybe you could mock up what you’re imagining the addon download process would look like. Since it would have to be optional and not introduce an extra step, I’m not sure you would get enough data to warrant a change.
Maybe a “Enter your email address to get updates about this module” field combined with a new feature to send out notifications when a new version is uploaded and/or an easy way for an addon maintainer to send a message to all pepole following an addon... though, anyone with an OpenMRS ID would expect this to be a button tap (not enterIng an email; rather, just sending to their OpenMRS email address on file).