We heard about NVIDIA using GDDR5X on its next-gen Pascal-based offerings, and shortly after it was rumored that AMD would also use GDDR5X on their forthcoming 14nm-based parts. A new report has surfaced about GDDR6 arriving in 2016, which could be a rebranded name for GDDR5X - before it was even allowed to be released.
It makes sense though, as some users might think an 'X' isn't enough of an indication of increased performance between GDDR5 and GDDR5X, with a full number increase from GDDR5 to GDDR6 being a much better marketing strategy. The new GDDR6 is under development at Micron, but we don't know anything on the performance or specification side of things. We do know that the jump from GDDR5 to GDDR5X was actually quite significant, and while not quite as revolutionary as HBM, it was a great jump for cards not based on HBM.
Considering GDDR5 debuted on the 60nm process all the way back in 2007, it has matured greatly since then, where it is down to 20nm on current cards - including the GeForce GTX Titan X. I think GDDR5 and GDDR6 could be used starting next year in cards that are under $400, allowing the rest of the market above $400 to use the much more expensive HBM2 technology.