The biggest rumour is that The Witcher 3 DLC will take place in a Dune-like desert region called Zerrikania, and would be similar in size and length to Blood and Wine, the last DLC for the Witcher 3.
It seems DLC for The Witcher 3: Wild Hunt has been all but confirmed after another report has emerged claiming it's happening and fresh details about what players can expect have been revealed too.