Alright, I feel they were mostly fine, especially the backstories, but some things I would change by character all knowing I have 20/20 hindsight.
Blackwall- Many Blackwall fans may disagree, but I found it disappointing. He was too safe. He did bad, but just enough to be bad and be forgiven. Also, his evil self had no tie to the plot. It would have been better I think if he was just a bit more ruthless before. Also, if he could be someone other than a Thom Rainer we just heard of it could have been a much more intense scene.
Cassandra- I have to admit I dislike little about her plot, and much more about her portrayal for entirely personal reasons. So I will just say I liked her a lot. Maybe less yeah you may not like being the Herald but too bad you have no choice.
Cole- I really would have loved a scene where he actually was almost turned into a demon, and that spurs his quest for one way or the other. I dislike it relied so much giving us what he did in the books. Too much of non game material was part of this game and it hurt him being a companion when taking that book as truth. He already did his journey and it was practically completed before we had him, and since Cole as a character doesn't speak clearly, any talk on his past was lacking. Some things like how he was a demon and not a spirit would have been nice, or even dealing with him without all the baggage of the books.
Dorian- I think a lot of people love Dorian because he was done perfect. He had a stake in the plot more than most, he was involved, he was complex, and thank god he actually explained himself unlike most of the characters in the game who always seems to half talk so there was never an explanation. If anything, I find it frustrating that perhaps the first Dragon Age character where the correct answer is being reconciled with his family, is him. The correct answer for Dorian according to ambient conversation is reconciliation which is a bit messed up, also it is the only way to continue the scene. I would have preferred a bit more confrontation with the father, and less like it is an intervention.
Iron Bull- The problem with Iron Bull was the Qun was so far away. It would have been nice for one Qunari camp (like the dalish camp but all Qunari soldiers) could have solidified his presence. The game was a bit of a data dump at times so if he could be centered outside of Lore as to what he was part of it would have been nice.
Sera- She talks in riddles, and I love her, but for players you sometimes need to spell it out for them on what and who a character is. Everytime she explains anything it is more "I don't know I just do" also, perhaps a big part of her life was very barely explained by her relationship with her "mother figure" She just learned how to shoot. She just started up with being a Red Jenny. And the Friends just sort of work. Her entire character would work better is she actually made clear to the player what she is about. Personally I think she is an Elven God, but if she was so mysterious she could never be clear about her past, then she may have been better as an NPC. Also what is with the pathological hatred of elves?
Solas- He made the game anticlimatic and he rarely explained what he was about. The more we learn the more it feels like the character was a puppet and he was pulling the strings. Maybe explain things. At least explain how the plot is working. Some things like the Well and Abelas was meaningless without more context. He was there to provide more context but often made it more surreal. Like Sera, be clear or be a non companion. The fact he also disliked the Dalish just made it difficult to take a major part of the world seriously.
Varric- His heart was with Hawke and he never really explained what is with him and Bianca to the point that as much as I like him, he didn't add much to the game. He was there to talk about Lyrium and Hawke (who was part of a different game) and he didn't really add anything to the game itself.
Vivienne- She could have been much more than the talk on how mages and Templars should act or not. Too much of the game were characters subbing in for codexes or plot dump. Vivienne wasn't allowed to be a full character. Not enough with her lover, not enough to cement her identity outside a mage who has ideations of political power. She often seemed onesided and can't help but feel she lacked characterization in general. Anything, preferably stories about the court both ludicrous and tragic would have been nice. Also humanization outside being a madame de fer would have been nice too.