SignificantTermsQParserPlugin is not yet visible to the users (was not documented or spelt correctly in 7.4), so there is still a chance to fix its output before people start using it.
Currently, it injects 6 different keys into the document, on the same level as responseHeader and response. This feels like polluting top-level space. It may be better to put all those keys under one bucket (e.g. significantTerms). Additionally, resultCount is always the same as response.numFound (documents found), so does not seem to be needed.