If we're discussing future GPUs, does anyone know what exactly AMD means by "next-gen memory" when it talks about Navi?

At first I thought it means that HBM2 would be available across their whole lineup, but I think if that was the case they would've just said so.

I've also read a bit about how Nvidia wanted to go with HMC (Hyper Memory Cube) instead of HBM, but it seems it's still at least twice as expensive/GB, and they needed to go with high-end GPUs that have at least 16GB. HMC is not even at 8GB yet. Intel also seems to have adopted HMC for some servers.

So is the "next-gen" memory HMC, or something else? AMD is supposed to come out with it in 2018, hopefully at 10nm.