Yeah they said Terror will be coming first.
Terror Storymode will drop Campaign/Blackhole
Terror HM will drop the New tier 63 items.
And yeah EC Nightmare will also drop 63 items and will be used as an alternative source for the next Set, as well as being a challenge.
Edit: Also received a tweet from one of the Community team saying there's no news yet :< Hope we hear something soon!
I have not read anything to confirm what you are suggesting. I think it's plausible, but we don't know any of that yet.
edit: The only thing I recall reading is that NiM EC will help with obtaining the new set. What that means though has never been dissected.