DLC has been a bit of a tricky thing for the Assassin’s Creed series because it’s so story driven and there’s a lot of canon to deal with, both in the game and historically. AC3 tried to mix things up with a massive what if scenario involving George Washington as the bad guy, but the episodic release cycle had people (including myself) losing interest before it even concluded.
AC4’s first significant single player DLC, Freedom’s Cry, seems much better planned out and well executed by contrast. First of all, there was plenty of setup in the main game by making the DLC’s protagonist, Adewale, a major NPC as Edward Kenway’s right hand man and counselor, and then setting the game 15 years in the future to keep needless crossover out of the way. Secondly, Freedom’s Cry delves into a very different but extremely significant theme of the seafaring era, slavery, and does so with the right amount of respect, while also allowing to indulge in the aspect of righting the terrible wrongs surrounding you. As the game progresses you quickly find that Adewale is a powerful, intelligent character who should command more respect than Edward but doesn’t, merely because of the color of his skin.