Mixed feelings:
It was obvious to me we would see him again.
I think he was fine as the Big Bad for DA I, at least in theory, but I really wanted him to be a lieutenant. Maybe even for the big bad for DA 4. Either way works, but I wonder.
Even as the Big Bad in Inquisition it seems he had wasted potential. Again, fine, and I do not think I dislike it as most people, but could have been a lot better. In Legacy he was so compelling and such a lore bomb, the accent, the confusion, and the sense of something terrible waking up. It would have been a great opprotunity to examine two of the more fascinating aspects of DA lore, to me. But...didn't really happen.
It could have been a great examination of faith and conflicts between religions, and they touched on it. But could have really gone into it much more in depth, I feel.
Yeah, in the run up to Inquisition I was certain he'd be back as an enemy, but I really didn't think he'd be the actual main bad guy either. I assumed he was the Darth Vader to the true Big Bad that was the Emperor, the physical threat and muscle that had to be overcome to defeat the chessmaster who was on the throne and pulling strings behind the scene.
It's not to say that Corypheus isn't a palpable threat, because he is and learning that he was the ringleader of the Seven Magisters who broke into the Fade does back up that he's got the villain cred, but the problem is that Corypheus the mastermind and Corypheus the person seem to be two very distinct entities, which are really hard for me to reconcile?
From his manipulations and machinations across Thedas, we see some of the intelligence that would conspire to sunder the Heavens at work, someone who's very adept at taking various power-hungry people and playing them against each other to leave them with nothing, while he gains everything instead. He was able to manipulate the Orlesians, the Templars, the Mage Rebellion, the Venatori, the Wardens and countless others to do his bidding, while leaving both his existence and true plans completely unknown to all of them, showing he's smart and savvy enough to be a master of the Game.
And yet, in person, Corypheus seems more like a generic villain who throws a temper tantrum whenever his plans are thwarted.
When he loses the Mages/Templars and the Breach is sealed, his petulant attitude causes him to attack the Inquisition directly in response, ruining all the advantage he gained by keeping his existence hidden and a mystery to everyone. Furthermore, he only ends up strengthening his enemy, as the survivors are convinced afterwards that the Inquisitor is truly a holy figure in how they were able to stand up to him and his dragon, then survive the mountain crashing down upon them.
During the Well of Sorrows, his reaction to being thwarted yet again by the Inquisitor and losing what he sought, is to charge at them roaring in rage, then get an even worse brusing to his ego all that accomplishes is face-planting into the Eluvian when it closes in his face.
(And as an aside, does anyone else have the mental image of him dusting himself off afterwards, picking shards out of his face and hoping to Dumat that no-one from his side saw that complete and utter fail?)
Then he responds by simply opening the Breach in order to draw out the Inquisitor, despite having absolutely no reason to do so other than because he wants revenge. As your advisors and Solas note, the smart thing would be to lay low for a couple years and rebuild his forces. He's a darkspawn and thus biologicaly immortal, in addition to having a Dragon acting as his soul jar to keep him from being slain, so it's not like he doesn't have the time to do that... but he doesn't?
I suppose, the sort of madman who wants to break into the Fade and has done so now on three occasions, isn't the sort of person lacking in ego and likely isn't going to be someone who enjoys getting said ego repeatedly kicked in by an upstart like the Inquisitor? And I suppose having that sort of power is bound to make anyone a tad overconfident and dismissive of anyone as being a threat to him?
But even so, it's still hard to see the brilliant mind that supposedly concocted all of those wonderful and dastardly plans we foil across the game, with the guy we keep bumping into who throws a wobbly when things don't go his way, he's highly untactical when it comes to managing his troops, (only when he loses the Wardens, does he start to treat his troops with more consideration and tell them to not zerg-rush anymore) and enjoys villainous monologuing during battle?
I'm not saying he was a bad villain, but it does leave me thinking that he really wasn't the mastermind he's made out to be and that in reality, Corypheus relied a lot on his various henchmen to do most of the planning for him. If the Architect was indeed on the magisters who broke into the Golden City with him, it makes me think that the Architect and the other five were probably the brains of the outfit and Corypheus was merely the pointy-haired boss who barked orders?