To have a menu item on the client portal that allows the client to set their own marketing consent options for GDPR would be very useful.
We have them in our engagement letters and have them set as "no marketing" as a default, but allowing clients to do this when logging in for the first time, having the option to change them and reminding clients that have not renewed them periodically would be good for GDPR compliance.
Finally, whilst discussing GDPR, having an option to remind the Data Controller to delete information at a certain date, with a task for the Data Controller to take action would be very useful.