As opposed to mediawiki, where all citation data are stored as strings, Wikidata has a strict typing system. https://www.wikidata.org/wiki/Special:ListDatatypes
Done:
[x] New wikibase format added and deployed
[x] isbns with dashes - deployed for all formats, see T230057
[x] Seperate properties for ISBN10 and ISBN13, deployed for wikibase format
[x] Identifiers key: added, see T245142
In progress:
[] pmcid without the pmc prefix - part done for all formats, see T157152
[] Identifiers key: added and deployed for some identifiers, T245142
To do:
[] Hierarchical structure for containers: T245142
* Plausibly a structured date format compatible with wikibase dates. Failing that, validate all date fields and return ISO dates (currently return unvalidated strings which may be invalid for wikibase).
* Validating language codes against wikibase's allowed values. see also T217258
* Plausibly including language code with every monolingual type, if possible. see also T217258
Alternatively we may even consider a wikidata format, which will actually try to obtain the correct QIDs in the background for certain params.