There's a rumor making the rounds that there's a successor to GDDR5 called GDDR5X and that this new memory technology will debut with NVIDIA's Pascal GPU lineup. If the information is correct, NVIDIA's high-end Pascal GPUs like the GP100 GPU will use HBM2 memory while low-end and mid-range parts will use the cheaper GDDR5X memory.
TweakTown heard GDDR5X will stick to a 256-bit memory bus and will offer memory bandwidth of up to 448GB/s:
The latest rumor is that there will be a successor to GDDR5 called GDDR5X, and it will debut with NVIDIA's next-gen GeForce product. GDDR5X will reportedly keep the same 256-bit memory interface we have on the current GTX 980 while ramping up the memory bandwidth to 448GB/sec - higher than the best AMD cards, save the Fiji-based HBM-powered cards. HBM2 is still obviously on the cards, and will include a 4096-bit memory bus at 1GHz with memory bandwidth hovering at 1TB/sec.