The legacy driver would return the inserted index as a bson document. mongoc_collection_create_index should exhibit the same behavior.
This will require an enhanced new function, mongoc_collection_create_index_2 or something, that returns additional info.
History: we created mongoc_collection_create_index_2, renamed to mongoc_collection_create_index_with_write_concern, and now we're renaming it mongoc_collection_create_index_with_opts. All of this is for 1.5.0.