Hello,
We want to provide a "preview" feature in the email but it requires data to display, so an order for order notification, etc.
Because a lot of specific case exist, we can't provide a "sample order" for the preview and forcing the selection of an existing order might not be the ideal. So we are still discussing how it could be implemented.
That's why we mostly recommend to test your modification in the "order notification email" which is the used content when you click on the "email" button while editing an order in the backend.
About the WYSIWYG editor, due to the tags we use in the emails, it's hard to have a compatible editor.
More editor remove the commentaries and will display errors regarding the tags.
It would require to create our own editor ; like Acymailing did and it's a really huge development (it took at least one year for the AcyEditor).
So yeah, we'd love and it's in our TODO list. For the moment we have other tasks but you can be sure that we want to improve the emails. In HikaShop 3 we added the support of templates (which made easier the global customization of the emails for the color, header, footer, logos, etc).
Regards,