@Erica, I'm not sure I understand.
Does "One way formatting via markdown, ..." imply implementing the same formatting mechanics as seen in Dropbox Paper and Quip in such a way that the markdown syntax is only used as 'shortcuts' to implement the style/commands under the hood?
If so, then I wouldn't personally be opposed to that as long as performance isn't compromised for larger and heavily nested lists. However, the only condition would be such that the WYSIWYG mode remains still somewhat "à la Markdown" in the sense of only very select formatting options are available and apply simple rules (i.e. limited).
For example, font style and size selection are not a rich text feature (you can't mix and match Arial and Helvetica with 12 and 16 pt font within the same file - these are the sources of pure evil within gDocs, mWord, and any more complicated document processors).
Also, currently the headings are implemented somewhat WYSIWYG mode, assuming because of potential tag conflicts, is there any use for them other than stylistically (bold and larger)? Like, are they used to anchor which items to appear in an outline in the side pane for example (I'm pretty sure that's not the case right now, but that would be very cool).