It seems the sibling episode from DA2 is main focus of the discussion. Based on these pages of discussion I actually see two seperate questions in relation to companions influencing story:
1) If companions influence the story should a hint be given beforehand to the player, so he or she knows a certain companion will have an effect?
2) How big a hint should be given? (this is basically the debate of DA2 going on right now)
How about a made up example in DAI regarding the first question:
At the Keep the Inquisitor plans out his next venture out into the world, as he prepares to leave, Dorian comes up and ask to join since he knows some of the people involved in the area. The player now decides to take Dorians advice and brings along him, Vivienne, and Sera.
Completing the quest three consequences happen:
a) Because Dorian accompanied you extra dialogue was available and you were able to resolve a bad situation peacefully and now both factions involved in the quest are open to further exploration, quests etc. If you hadn't brought him along combat was bound to happen, though the quest would still be resolved but with only one faction surviving
b ) Because Vivienne was there 2 side quests are now locked out for the rest of the game even though you had no forwarning of this effect. Had you brought any other companion everything would have been fine and the 2 quests available.
c) Sera dies no matter what choice you pick during the quest. She is scripted to die simply by you bringing her along even though no forwarning was given. Had you brought along any other companion, this would not have happened.
So the question is which of these three are acceptable, all three or none? As I read the OP the first is fine, while b and c are not ok, which is my personal opinion as well. Naturally we as humans have different opinions on this, which is ok.
The second question is way tougher to answer as that is a lot more subjective whether the hint was clear enough. Here we just have to believe Bioware hits the best fit for all.