Pixomondo has released a video reel of Game of Thrones VFX, this one focusing specifically on the process that brought Drogon, Viserion and Rhaegal to life for season 5.
Werner Fuchs (German agent and friend of GRRM) confirmed that GRRM told David and Dan what was going to happen in the Weiter two Bücher in case that he shouldn’t be able to finish the books.
"The Drachen mature this season so their look is Mehr fierce, We changed the proportions up a bit and made the spikes Mehr prominent so the Drachen are much Mehr menacing now."
This year, due to increased VFX work, these two smaller studios have had to bow out and Thrones producers went searching for a larger VFX studio to take over. It appears they have found their replacement.