Fundamentally speaking, I feel Thedas is as safe from Solas as it was from Corypheus. Unless they make a hard turn with the story and change the reality of DA, I'm not overly concerned about will happen. I feel it would be something to the effect of the Mage Rebellion, which was major and unstoppable but not "doomsday." If I'm supposed to be afraid of just the foreboding idea of Solas' plans... Well, I'm not. I think I worry more for Solas himself than what he thinks he can accomplish? I don't know. The threat just doesn't feel that real to me yet.
Yeah, I honestly don't think he'll succeed. Dragon Age simply isn't that kind of game. But I have a feeling it will be more cat and mouse, or chess playing, as the new DA4 protagonist faces off against Solas. Solas isn't a black hat wearing villain like Corypheus or the Archdemon/darkspawn h orde was. He's more nuanced, like Meredith was, doing what he feels needs to be done and limiting the collateral damage as he can. Which makes him far more dangerous.
Like thats1evilguy said, it would be a very different game if they found a way to make it personal for the protagonist. It makes me wonder if the main antagonist will be some intermediary, like some Solas lieutenant, or perhaps a secondary bad that rises simultaneously, with the menace of Solas hanging over the whole game. I honestly don't expect Solas to even really show up until the end, like in Trespasser. Then, my Inquisitor damn well better be there!!! 