Hi , have you noticed if you drop html into the email activity in a workflow, it sends the mail as rich text?  I don't think it's documented, but I've used it and it worked.  The only thing i've noticed is the mail formatting appearance appears differently when viewed say in outlook desktop v the cloud experience but i think that's an outlook thing.

That said, a proper HTML editor in workflows would certainly help the appearance of outbound workflow messaging.

In addition to this, a rich text editor would be greatly appreciated in workflow email tasks, like it is available through business processes.