We want to structure the content in our "notes" by adding custom fields. We want to do this in notes because it is the only place where you can shield datafields form other users.
Its possible to add custom fields for Notes through UI, by simply adding an option for
Objects a custom group extends to option group.
However the fields wont appear on Add/Edit note form unless you write an extension to build the builds or modify core file to support custom fields(should be couple of hrs of work). Good news is the custom field data can be stored/viewed against the note by api.
There are other ways to make data more private, eg use an Activity Type and limit certain Activity Types to certain users (eg Private Calls/Meetings) via this extension
The privacy on notes fields is not very flexible. You can control access to a set of custom fields using ACLs.