Crusader Kings II: Way of Life

Brings a much needed depth element to the game. Instead of making the map bigger or the timeline longer, accent is put on the "story" you are creating for your dynasty. An excellent expansion, the Focus mechanic will be useful in every game of CK2 you play, and all 10 of the Focuses have their advantages depending upon your character's circumstances and your play style.
Also it allows you to own a dog. What more do you want???

This expansion mostly expands the roleplaying in CK2. Some people are mad about the features locked away behind DLC, but if you can get it for cheap it's worth it. Either that or go for CK3. Both good options, depending on your situation.

For those seeking more roleplaying options and to give more flavor to all characters, this DLC should be part of your library.