For example the worker related properties should be split among those which are global and those which are applied to the specific backend executors.
Also we might want to rename indexing.synchronization-strategy to automatic-indexing.synchronization.strategy, to be consistent with lifecycle.strategy in the Elasticsearch backend. Same for other strategies, for example multi-tenancy strategies.
Also, as mentioned here, we might want to add prefixes for configuration properties that are only valid for a certain type of backend/directory/... . For example:
hibernate.search.backend.myBackend.directory.root would become hibernate.search.backend.myBackend.directory.local_filesystem.root
hibernate.search.backend.myBackend.directory would become hibernate.search.backend.myBackend.lucene.directory
and so on.
This has the serious disadvantage of making the property longers, though, and the usefulness of this added verbosity is doubtful, since a backend/directory/etc. only ever has one type and only ever accepts a single set of sub-properties. I would be against that, but let's discuss it if there are strong arguments in favor of it...