First of all I want to say that I recently started using Joplin after covering it on my podcasts (1, 2) and I am now a big fan of the project! It is a great solution for notes and todos that I have switched almost everything over to Joplin and plan to completely migrate soon.
I would like for you to create Keyboard navigation for the menubar ("File, Edit, View, etc")
For example, let's say I wanted to import a Markdown Directory that I created as a reusable template. At the moment, I have to do it with the mouse and that's fairly slow.
Instead, I would like to be able to press Alt+F to open the File menu, then I (for import), then D (for Markdown Directory). This would turn the process into something that takes maybe half a second.
I do like that a lot of functions have support for specific keybindings but not everything works like that because with the Imports you would have to choose which form of Import you want and Keyboard Sequence shortcuts via Menubar Access would facilitate it nicely.
I would also suggest making it navigateable via arrow keys. Left and Right from main menu sections or to activate submenus. Up and Down to navigate the dropdowns.
This would also solve the issue of not having a shortcut for making a new Notebook. I actually make these all the time. At least, once a day but usually more than that.
This would make it possible to have Alt+F, then B to create a new notebook. (or Ctrl+K could work for that but still would prefer the sequence method)