Ability to add a private note/comment (only visible to Givebutter admins) to a donation and not just the contact's account profile page. This could easily be done by allowing offline donations to have the "message" field include a padlock icon to lock / unlock visibility to the actual donor OR could be a custom text field with the ability to toggle off if field should display to donors.
Additionally, since this Notes field may include pertinent information about that specific transaction, being able to include this field as a column in the .csv export would be necessary.