Things that publish fedora-messaging messages are supposed to provide a message schema:

https://fedora-messaging.readthedocs.io/en/latest/messages.html#schema

the benefits are as described there, plus a schema will be necessary to replace fedmsg_meta for purposes like providing a summary of the message for notifications via FMN or in the datagrepper view. waiverdb does not currently do this, it's just using the generic Message class.