I use tags as an easy and fast method to categorize notes, but i often wish for a method to assign subcategories, like with a class based system. This would allow for nesting and easy filtering.
This way i could assign "Berlin" tag "city" and "Germany", Germany has the Tags "country" and "europe".
"Barcelona" would also get "City" and "Spain", with Spain getting "country" and "europe".
Europe would inherit "Earth" and so on...
Now you can easily search for all cities or countries in europe, or all cities in "spain" or even all over the world.
Not sure if this is even technically possible at this stage of development, or could be achieved differently - but it's a highly practical feature i yearn for.