Implementation ideas: perhaps have a DOI field that, if empty, triggers a checkbox that says "Mint DOI?" and, if checked, triggers an action at the Drupal level that farms out the DOI minting request to whatever vendor backend you have configured, and sticks the newly minted DOI in the DOI field. This should be seamless from the user's perspective, but completely configurable from the repository administrator's perspective.

@mjordan's 7.x islandora_doi_framework module provided an abstract DOI handling API and put the DataCite API specifics into a submodule so that the main module could be extended to work with other vendor backends through submodules as well (like CrossRef). This should be like that. Drupal 8's search_api and search_api_solr are another example of this pattern in practice.

My specific use case is for DOIs, but if we are trying to make this an abstract API, would it make sense to start at the level of generic identifiers that need to be minted from a third party? This would allow for the use of other DOI-like identifiers (like ARKs and Handles) too.