I had a similar idea, but it didn’t go very far beyond research. There are some special app interfaces that people have developed that remake email to look more modern like chat apps or social networks, by removing all the boilerplate.

Some of the issues I was thinking about:

Email clients by default block many types of messages and the allowed mime types are limited as is the support of html. So you really need your own email client to bring in the types of features we’d like to see, or, as you say, an intermediate format that is reinterpreted.

There’s also the fact that gmail or outlook mail servers may simply block and blacklist the content. Email was designed to be decentralized but it has moved to a system where a few companies control the major mail servers. If you wanted to re-decentralize email and add some anonymity then everyone would become their own mail server but this would raise the problems of email viruses and spam - and it’s not as convenient as just using your existing email and app.