Both Sebastian and Talis add depth to the cultures in the storyline and a few to several hours playtime, but Hawke's lineage quest is a keeper - better than the game in my opinion. Also most of the armor/weapon DLC depending on play style.
My fundamental problem with DA2 was the tiny world space and poor graphics of the core game -it was a quick release in a small package. However it was clear that inquisitions was in the works from the cut scenes. Most of the DLC has more refined graphics and world space, but most people would agree that Talis "herself' was poorly done.
DA2 is Hawke with sister Bethany, carver just got on my nerves. A simple "runscript chargen mage" code can do it at level 2. It works fine in all the DLC and only a few early ghostly carver scenes in the lowtown home. You may need the black emporium to change your look also. There is probably a code the change character settings, but I'd have to peer into the DLC to find it, if not on the web.
I have all the DLC and just run it for notes and to answer questions about the decision tree and spells like healing in the creation tree that may be missing in inquisitions, but mostly for Sera of inquisitions - I have high hopes for that little elf to have a lovable personality and quest aware behavior and body language. It all depends on the success of RPG modeling to frostbite, but Sera has a lot of dedicated support, probably more than main characters. They will use DLC to import some or all of her if necessary! That said I'm not impressed with the generic character details to date, as frostbite 3 is capable of hyperrealism NPCs with intricate detail and most seem way too cartoonish so far set in a spectacular "living world" space.
I already have requests to mod the game and its launch is a month away - that can't be good sign!