The standalone DLC for Total War: Rome 2, Caesar in Gaul, is out December 12th for $14.99. You can find all the details on the Total War forum, but basically, the expansion focuses on the Gaulish campaign as the title suggests. It reminds me of Total War: Medieval Viking Invasion, which focused on the conquest of the British Isles.
Here are some of its features, which surprisingly are not convincing selling points like the Blood and Gore DLC of which the title itself is a selling point:
– Time-span is 58-51 BC and each year consists of 24 turns. Seasons affect your harvest, which affects your men’s bellies, which affects your ability to slice open your foe’s bellies.
– New historical battle: Alesia. That was a major battle but it’s a siege battle, and siege battles are not the most interesting battles to play in Total War. But it would be satisfying to see Vercingetorix whip Caesar’s ass.
– New campaign map with 18 provinces focusing on Gaul.
– Some new playable factions and new units none of which I found particularly interesting or new. 
I bought Mongol Invasion because I wanted to command Koreans against the Japanese. I bought Viking Invasion because the Irish deserved to conquer Britain. I bought Barbarian Invasion because I wanted to see guys swim. And so on. I’m sure there will be those who find the new expansion appealing, but based on the features, which appear to lack innovation and creativity, it appears Caesar in Gaul will not add real value to the brand.