I actually have to proposal regarding font settings for the editor.
CSS-like editor font selection.
Now, as far as I have tested, I can only select one font for editor. Which is often not ideal. My native language is Bengali which doesn't use the Latin alphabet and systemwide Bengali font is not good so the font Joplin shows is actually not much readable. Of course, I can choose a Bengali font in settings but then I'll loose nice monospace Latin font. A CSS-like font-family definition will allow us to select fonts and fallbacks more efficiently.
Different fonts for WYSIWYG and default markdown editor.
It can be made optional and as a overriding field. WYSIWYG editors are ideal for less-technical users and they often use it to write non-technical things and these users don't like monospace always. This feature will will be very helpful in my opinion. For example, sometimes I write purely technical things, mostly in English and find markdown editor extremely efficient. But sometimes, I write long articles in my native language for which I have to use external editor like Typora. This feature will eliminate the need for external editor at least for me, and I beileve for many others.