askvortsov I made a very simple awful looking checkbox to toggle between WYSIWYG and Markdown/BBCode. There's one issue though - there will probably not be space for more buttons in the toolbar. Could be fixed by making the list scrollable and putting the checkbox to the right of the composer window.
Desktop:
Mobile:
Although, I suppose that the preview "eye" button's functionality could be replaced with the toggle WYSIWYG feature. It makes sense, as WYSIWYG is basically just a "live-preview" of markdown - and still, if people want to see how the post would really look like, they can minimize or close the composer.