That's right, we still need to see an actual 300 series first of course, but there already is talk and speculation about the successor architecture from  AMD, a product which carries the label Greenland. Fact remains though that other than this GPU/IGP codename and the usage of High Bandwidth Memory (HBM) the details remain scarce. 

Greenland is expected to launch in the 2016 time-frame and actually will be integrated into the next-gen AMD K12 APU, series 400 that is.

"Details are limited, apart of the fact that Greenland can end up in the next generation APU such as K12, making the architecture quite scalable. High Bandwidth Memory combined with new K12 cores might create the fastest integrated product of all time, and let's not forget that AMD is putting a lot of emphasis on Heterogeneous System Architecture (HSA) and the compute side of things. With the help of HBM-powered Greenland that can end up with 500GB/s bandwidth, along with multiple Zen 64-bit CPU cores, you can expect quite a lot of compute performance from this new integrated chip."

The rumor originates from Fudzilla which mentioned that the Radeon R400 series will be called the “Arctic Islands” family. They do not mentioned which graphics cards or branding the Greenland GPUs will use. It is also mentioned that AMD will be launching Fiji GPU in late June or Q2 2015. Greenland could end up in the K12 ARM and x86 Zen core architecture (servers) which are due in 2016 as well. 

AMD prepping Greenland Graphics for 2016