One of the highlights of the expanded universe has easily become Cartoon Network's Star Wars: The Clone Wars. The series started of a little slow in my opinion, but now that I look back on it, it was definitely done for a reason. The relationship between Anakin and Ashoka needed to be cultivated and groomed so they could trust each other going forward.
The other important part for the previous 4 seasons has been to show the relationship between the Jedi and the Republic, and how it will become strained as time goes on and the series pushes towards Star Wars Episode 3: Revenge Of The Sith. From the look of this trailer, season 5 will be a lot darker and looks to be completetly awesome:
As you can see, Darth Maul is back and will be in a story arch, so are the Mandalorians. It looks as though there will be a huge story arch involving the liberation of an occupied Alderaan. There looks to be ore death, and the enemies of the Republic seem to be gaining steam in the fight which leads me to believe that things are going to come to a head this season. I would not be surprised if there are only 2 or 3 seasons left.
Season 5 starts this fall on Cartoon Network.