I would pay someone to write a comprehensive guide to all meaningful, useful, and overall interesting quests in this game.
For example (and probably one of the most obvious) - Crestwood's "Still Waters" quest. That had an interesting story behind it, as well as some interesting side quests you found along the way. I particularly liked having Cole along for that one.
I'd love to know all of the quests that lead you to interesting locations like Valammar (in the Hinterlands):
I'm sure there are other locations like that in the game that could easily be missed. That was tucked away behind a waterfall, and I had no quest to take me there. I stumbled upon it by accident (coincidentally looking for treasure behind a waterfall, but this was the wrong one).
Some of the areas are just too big to explore and find all this stuff