In what ways are you considering changing the metadata model? Are you considering only a fix to the immediate problem with the groups system?
The ACS4 requirements and design docs talk about 'modifiable data models' but the implementation seems incomplete. For example, one of the goals of such a system should be to illiminate DDL at runtime. However the acs_object_types table has not null and unique constraints on table_name, which forces a new table to created for each new object type whether it's needed or not.
Object types themselves are not acs objects, so custom attributes can only be associated with specific instances of objects, not every instance of an object type, which is the more useful scenario. When you have users creating new object types from a web interface, you start to want to use the permissions system, again requiring object types be acs objetcs.
I think the lack of this working feature is quite an impediment to the system. It should be used by many modules e.g. groups, ecommerce, simple-survey, cms, but instead each module has to re-invent it's own solution.
Obviously you're on a schedule so I don't know what it makes sense to do before launch.
