Neither Sera nor Vivienne change. Solas doesn't either. He has more going on than Sera or Viv but he doesn't really have any growth. Now that I'm thinking about it, neither does Varric.
Don't agree in regards to Solas. The Inquisitor can either make him reconsider his views or else reinforce them. For a character like that even to consider a romance is already a big development for him. I guess you could say the same of Sera and an elf.
Nah, my Lavellan is always happy to take risks if it means improving life for the Dalish, but I'm still pretty sure she'd hear "I'm going to tear down the veil" and be unable, or unwilling, to see it as a positive move, since the only evidence she has of a missing veil is the awfulness that came from the breach.
But Faded demonstrates that Solas is right- a spirit forced to act against its purpose becomes a demon, but when that stress isn't there it remains intact. Cole comes through intact and his impact on the world is a positive one (with guidance). Ambitious madmen like Corypheus and the Venatori are able to wreak so much havoc because they're trying to control a scarce resource. Give the resource away to everyone and you take away that temptation.
I'm pretty convinced it's just wishful thinking on the fandom's part though, to imagine that Solas isn't going to be a major obstacle of some sort in future content. Yes, Tevinter, the Qunari, etc are still there for conflict exploration, but the final stinger does not tease a massive war, or a new threat in the form of either of those; it teases one in the form of our beloved bald apostate- or at least, that's what I read from it. Maybe we won't have to battle him, maybe we can choose to side with him, or maybe a new protag will just be left to clean the newest mess he made with his latest "fix", but I have a very hard time believing that we can expect a true "And Alistair and the Warden got married and became Queen and King, and now the Queen is even finding a cure for the calling so they can make babies and live HEA" type of ending for Solavellan. David Gaider simply loves our tears far too much to allow that kind of thing to happen.
Some people thought Morrigan and Flemeth were evil. Meanwhile the appearances of both Morrigan and Warden Alistair in this game were pretty sappy in regards to their DAO romances. Speculating what they'll do is pretty useless, though I guess it can keep us busy til we get a DLC announcement.