I've started reviewing some mail as plain text first. I've noticed some are exactly this kind of junk. The hard part of making a "new email" is that it needs to substantially better than current options.

Outlook still supports RTF. (I have no idea what clients support that) Any new format could also be included as a new content type.

For all the evils, I can't see any replacement markup being a significant improvement: What is the sender trying to communicate? Why is it beyond plain text? How do attachments not fill that gap?

I think the answer is: any client could choose to behave differently on the existing ecosystem. They currently choose not to. While an individual may think it's complex, the solutions aren't truly reducing complexity.