Unless there's another way besides using 'external link,' a way to add forum links to the navigation so URL's are updated automatically.

You can do it with Pages. Something similar could be worked out for Forums with a drop-down menu for selecting the forum you want to link. Include options of respecting forum 'read'/'view' permissions for showing the menu item.  

Similarly, allow us to select the Downloads app for a menu option with a drop down listing all of our categories.  

edit: Loosely related, allow selecting icons for menus (or a custom css class to apply to the menu header for the icon).

Agreed! I was just searching the forum here to see if this option was available. Seems odd that you can't link to a specific forum.

Minor bump.