While I have this context-switched into my head…
http://todotxt.org/ is a good example of how UI/UX might work for more advanced TODO management (including due dates). I’m not suggesting that Joplin should implement 1:1 all the features that todo.txt has, but the todo.txt community has a number of GUIs for desktop, web, and mobile (scroll down on the linked page for a list of such GUIs) which might serve as inspiration for how due dates or related features might work.