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 siguiente two libros in case that he shouldn’t be able to finish the books.
"The dragones mature this season so their look is más fierce, We changed the proportions up a bit and made the spikes más prominent so the dragones are much más 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.