With Empire at War, Petro had alot more work to do - which mostly focused around creating the game engine. With those foundations now down, they can tweak and improve it greatly, giving them more time to work on the actual game design etc.
Remember, you can take part in Petro Game days if you check the main page of the site for the time and date, which is around every other friday. I know Petroglyph are extremely keen to build on the success of Empire at War but the final decision is with Lucasarts.
All i can say is, watch this space as we will be first to find out when something is announced