Allow registered end-user to update entry, but limited only to specific fields (Craft 3)
You can include a custom Behavior like the ContentBehavior/ FieldLayoutBehavior in Craft 3 and handle those by yourself. This way you'll be able to handle permissions by yourself. There are certain Control Panel Hooks to include custom HTML.


Create a custom Behavior that's kind of a wrapper for the usual ContentBehavior
Store "special/extra" fields for those Elements in a custom table
Render those extra fields in a custom panel/tab
store those new values in your custom Table in beforeSave and afterSave funktions


But in general creating a custom Element Type would be easier to achieve, since you'll have more possibilities and more freedom to do what you want