Artwork by Chase Stone, from The World of Ice and Fire.
Interestingly, George also provides a update on Fire and Blood, the once-mooted "GRRMarillion" which was originally planned to come after the series was completed. This book draws on over 300,000 words of new material that GRRM wrote for The World of Ice and Fire, most of which was dramatically cut down in editing for the final book. With something like 80,000 words on the Dance of Dragons alone (from which the novellas The Princes and the Queen and The Rogue Prince have been drawn), this was always going to be a hefty project and, going through it, George and his publishers realised that it was going to have to be a two-volume project, with the first volume (containing well over 100,000 words, probably closer to 200,000) already effectively complete.
As a result, Fire and Blood: Volume I, being a history of the Targaryens from Aegon the Conqueror to Aegon III the Dragonbane, will be published in late 2018 or early 2019. Fire and Blood: Volume II, which will cover the period from Aegon III to Robert's Rebellion and the death of the Mad King, will follow several years later, presumably after A Dream of Spring is completed. This possibly means that the second half of the history will be a large larger and in more detail than originally envisaged, so for those who've been beginning for a prequel story about the Rebellion, this may be the closest you'll get (I also suspect this will have to follow Spring so George can give the full, "unedited" account of the war without having to worry about any more spoilers).
GRRM concludes his update by saying that we may therefore get two new Westeros books in 2018, if things go well (or maybe none at all, if they do not).