I was on Lamannia today trying out the new D&D tribute module quest 'Haunted Halls of Eveningstar' and the Thunderholme explorer area. Turbine will want to continue to build out the Forgotten Realms world as it was the most popular pencil and paper D&D campaign as well as the setting for a number of DDO's predecessors such as Baldur's Gate and also DDO's current competitor, Neverwinter.
The most likely place for the next paid content expansion would be Waterdeep. The 2015 expansion could be called 'Lords of Waterdeep' and then be followed by another tribute module from the Waterdeep lore in 2016.