I do worry that people will be creating Notifications with an identifier in particular.

Do you want to put identifier/expiry into the base notification? Even though the simple notification format does not output those values?

Also, I do feel many people will use Notification with those 2 variables. Sure seems like the base notification should be able to output an enhanced notification (or both)

Maybe the `Grocer::Notification` should be be the enhanced one and also add Grocer::SimpleNotification?