I don't see this as a straight add-on in Threshold. I think it will create other evolutions in the deck and at the end of them we will no longer have Threshold but something that is closer to the NLU that Nassif and others have been modifying.

Threshold as a concept is dead at this point. No cards in the new deck will actually have Threshold, only goyf will care about the graveyard (likely) and it will care about both graveyards equally.

Anybody who tries to play Threshold, as it currently exists, is likely to find themselves playing a tier 2 deck in a hurry. Lorescale Coatl adds into the creature mix alongside Tarmogoyf but he's going to be the better beater in the mirror and playing UG aggro-control competitively without him is going to be next to impossible.