Wikidata contains a variety of properties that refer to external resources using standard identifiers for such resources, e.g. ISBN, VIAF, MusicBrainz, GND, etc. It should be possible for clients to resolve these to URIs automatically. Team decision: We include the URI expansion in the JSON representation of the respective Statement, as a second DataValue in the same Snak.
Currently, Wikibase knows nothing about the interpretation of external IDs, which are stored and interpreted as plain StringValues. In order to provide URI expansion, a new identifier DataType would be needed on top of StringValue. The base URI would be recorded as a new "base URI" property in the respective property definitions (in addition to the URL pattern for the formatter; both can but don't have to be the same; team decision: no fallback for these two URL/URI patterns for now).