I really hope they do not bring back the League of Shadows or any connection to Ra's. To me, it would seem the easy way out in terms of plot devices. "Oh, lets go back to the bad guy in the first one, who has been forgotten about by the second movie, and play up that connection!" I feel it would make the series feel uneven with TDK the odd duck out.
Unless they retcon that Joker was released by the League as another way to destroy Gotham, which would be lame.