Heresy means to contradict the church.
Because of that, quite a few things were heretical and aren't anymore, or are heretical that used to be common practice.
Heresy is not a sin, it is irrelevant.
You're talking about blasphemy. Important distinction to make.
As far as your question goes: I believe you're asking, "If a book claims to be a 3rd testament, a New NT if you will, like the NT does to the OT, is that book blasphemous?"
Well, we have a real life example: The book of mormon.
And I would say, Yes. It is blasphemous.
There is no more to the Gospel, nor does their need to be more to the Gospel. The Gospel is Grace and salvation for any that believe. There is nothing to be added to it. Paul went so far to say that even if an angel appeared and told you of a "new" gospel, that he should be cursed twice over.
So reject any teachings that claim to be scripture that adds on to the Bible.