Server: Add support for tags
As the notes are stored as HTML the meta tag 'keywords' in the head section of the HTML could be used to store tags for notes. Tags could then be used for filtering existing notes. When entering tags existing tags should be used for suggestion lists.
This also can be a good idea to sort the notes vor ready/completed, important and so on.
Perhaps using the favorites tag from owncloud could be a good start, allowing to "pin" notes. For me groups (as it is now) and favorites would be enough, of course more fine grain would be great too, but I think tagging of files is also coming to owncloud, so perhaps better to wait.
Another alternative is to use subfolders for "Groups/notebooks" (unique) and store tags (multiple) within filename in sq. brackets.
+1 and the subfolders
+1 it is basically similar to groups, but allowing a note to be in multiple groups. It would also be nice if the Evernote import would include these tags (I have most of my notes sorted by tags).
It would be good to use the built-in tag system in OwnCloud/Nextcloud