I'm leaning more towards Valenwood. However, Black Marsh gets a very close second place because it would be interesting to see the implementation of spears once again wielded by dilligent lizardmen as well as dangerous and poisonous terrain..
Well TES:V was either Elsweyr or Skyrim. They did Skyrim. What do you think that leaves? While we can't be 100% certain until the announce it, (Obviously) I would bet $1000 it's going to be Elsweyr.