Intel has used eDRAM in the past. That's one possibility.
Original XBox One did this. That was AMD Bulldozer.
That said, the 6700's 6GB RAM amount is a little concerning. 5700XT 8GB is really a 1440p card. As the 6700 is supposed to be a 5700XT replacement, it should have a similar target window. You really will go over 6MB on certain games with the details turned up @1440p. 6700's 6GB RAM clearly relegates this to the 1080p territory for full details.
This caching system is a lot more effective than you guys realize, and will play a much bigger role when AMD starts using MCM's for graphics cards and possibly APUs. I had a feeling about this about 4, 5 years ago when I saw their architecture roadmaps for CPU's
First round of Navi APU's will use this cache with a separate MCM for graphics. The second round revision high end will include some form of 4GB HBM memory. It will be heterogeneous and part of the heap memory pool but reserved by the graphics driver.
My only mistake here was assuming the third round of APU's would be Navi based. I didn't think AMD would wait so long to bring the mods to the APU side because GCN is power hungry, and monolithic dies are not as cost effective (lower margins for APUs). I actually thought Navi would first appear in the 3000 series APU's. Looks like I was a couple generations off.