The ability to extend the durable unsubscribe functionality to additional fields.
Durable unsubscribe is explained here (https://docs.marketo.com/display/public/DOCS/Durable+Unsubscribe) and unsubscribes all records which share an email address, if that email address is unsubscribed.
The option to apply this to additional fields would be extremely useful in an instance where you have several brands (with a shared customer base) and are using custom unsubscribe fields.
With privacy laws focusing more closely on consent, being unable to set up this functionality with custom fields is a serious constraint.
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