The ability to set the following data retention preferences by day/month (ordered by priority):

All messages (i.e., the ability to set 6-month, 1-year, 7-year, etc auto-deletion of messages)

Attachments (i.e., the ability to have attachments auto-delete after 1 month, 6 months, etc)

The ability to easily delete entire message chains (and the ability for this power to be granted or revoked by an administrator on a by-person basis)

Signal-like functionality (i.e., the ability to set an auto-delete period for a given thread upon thread creation — something like, “delete 30 days after the last message”). This would be an incredible tool for many kinds of sensitive conversations.