I meant an open standard for apps-in-email, but then again, interoperability between apps is something that probably should be handled more generically at the OS level.
Coming back to my remark, email is one of the great federated protocols of the web, and I would really hate it if a big company would ruin it by adding proprietary extensions.
This seems to be extensions to the client, not to the messages themselves.
The extensions to messages they did was Email Markup, and they do use standard/open formats from schema.org (which is developed by multiple orgs, not just Google).
Coming back to my remark, email is one of the great federated protocols of the web, and I would really hate it if a big company would ruin it by adding proprietary extensions.