It appears that you are correct, Rabit, according to a poster on BGG:
To quote that poster:
What was announced:
8 new spirits!!! (4 with a brand new mechanic)
1 new adversary (beefy)
A lot of new aspects (some utilizing the new mechanic)
Only a couple major and minor powers (utilizing the new mechanic)
Some new fear, event (and I think blight) cards.All in all they said it is between the size of B&C and JE.
8 new Spirits is a lot more than I anticipated. We just got Jagged Earth (and Promo Pack 2) which gave us a total of 12 new Spirits. I would’ve imagined Eric would slow done a bit. (Not to mention the five new Horizons Spirits, for a total of 13 new ones this year.) (Just to be clear, I’m not complaining about this. : ))
So, it is confirmed that there will be a single “brand new mechanic” in Nature Incarnate. I feel like it is very obvious that it will be some form of the Vitality token (first seen on the 6-player play-mat).
But could 4 Spirits really use a single token in meaningfully different ways? None of the existing tokens are even used by that many Spirits. Wait, no, that’s not true; 
are all used by Sharp Fangs Behind the Leaves, Many Minds Move as One, Lure of the Deep Wilderness, and Grinning Trickster Stirs up Trouble. (And 
and 
both come close, with 3 Spirits each.)
Anyhow, of those four who will use the new mechanic, I’d wager that two of them will heavily use it, but in two very different ways, akin to how Sharp Fangs and Many Minds both use differently. And one will use it in a secondary capacity, similar to Trickster’s usage of . And the fourth will really just have it as a little incidental thing on one of its Unique Powers, like how Volcano Looming High can add and with Lava Flows.
I’m very glad that there will be “a lot of new Aspects.” I think that they’re a great mechanic, and it will be fun to see how Eric has changed up familiar Spirits.
And some of the Aspects will feature the new mechanic. Operating on the assumption that Vitality tokens (or something similar) is the new mechanic, I suspect that Vital Strength of the Earth, Keeper of Forbidden Wilds, Serpent Slumbering Beneath the Island, and Stone’s Unyielding Defiance are all likely candidates to receive it via an Aspect.
Of course, there is the possibility that the new mechanic won’t be Vitality-esque, or any kind of token at all. But then, I’m not sure what it would be.