Wikibase uses "vocabulary" URIs to identify things like reference globes, calendars, or units of measurement. It would be useful to offer a public API that could provide a human readable lable for such vocabulary URIs.
The FormatValue API module should support formatting of vocabulary URIs. This should be achieved by introducing a (pseudo-)data type for vocab URIs (using StringValue for internal representation). That data type would be somewhat complimentary to the proposed "ID" type: IDs would expand to URIs for normalization. Vocab URIs would be replaced by labels for display. Conceptually, they are quite similar though.