From what I know it is likely titled Forest of Darkness or Dark forest. It will introduce two new bosses (manticore and dragon) and will likely take place in a new area or an expansion of the existing forest.
Personally I think it could be (or I WANT it to be) about Artoria's, possibly giving more items with Artoria's and even more on gwyn's other knights information in their description. I'd love to see his armor, and even other additional armor sets in the game as well. Of course I think there should be some new weapons and an NPC or two.
Anyone else have hard facts, speculation or wishes?