This is truly just storing original document, right? And only returning the whole thing as well?
Because, in Elasticsearch, the _src field is actually used as source for several operations. For example, it is as a source for dynamic update as - by default - fields are not stored individually. And, I think, _src field also gets re-written/re-created on update, again because it is actually used as a source of truth.
The second issue I wanted to raise is how this will interplay with UpdateRequestProcessors (ES does not really have those). I guess URPs will apply after the content of the field, so the actual fields may look quite different from what's in the _src.
Finally, I am not clear on what this really means: all fields go into the 'df' . Do we mean, there is a magic copyField or something?
I think we need a bit more specific use-case here, then just an implementation/configuration. Especially, since a similar-but-different implementation in Elasticsearch does not fully match Solr's setup.