A magister who has - at least in part - achieved that "power of the gods themselves" he set out to acquire is certainly an interesting concept and worth exploring. The problem is just that we've seen him before and he wasn't that impressive then, and more importantly...
(1) He's been sleeping all the time and couldn't have had major influence on events beyond his prison.
(2) The few years he's been free are't believable for the developments we see on Thedas.
(3) Both Flemeth and Sandal allude to some big change and some entity that plays a part in it before Hawke encounters Corypheus.
Something clearly doesn't fit. Maybe it's an artifact of sequential storytelling and retcons, in that case it's not necessarily bad in itself but suboptimal storytelling as the writers - yet again - refused to be bound by past events to a degree I find unacceptable, or there's something important we don't know yet.
Well, if we're discussing spoilers and rumors...
I found an old rumor on 4chan saying that the villain was trying to tear their way into the Black City...
As for spoilers, Corypheus is depicted as having a special weapon in the Artbook, one designed to have a contrasting appearance to the Taint and the Dragon. The artbook doesn't explain what it is, bit it's described as "Pure and simple" and is a metal orb with a fingerprint pattern. There's a few other interesting concept arts, with one having Corypheus looking more symmetrical without any warped flesh or metal sticking out of him, and in a different outfit. Still Darkspawn...
Currently, I'm pretty convinced that he's our primary villain, but I think most of the real focus will be on the Black City and whatever he's trying to take from it or achieve by going back. Cory's interesting traits will likely derive from the fact that he's been to the Black City and learned something from it.
Those wanting a more alien and incomprehensible threat can probably look forward to the Black City. While Corypheus may be trying to use the City, we ourselves have little understanding of the place or its purpose. We don't know what could be hiding in it, or what would be released if it were opened yet again. Also, I would wonder what Corypheus believes his reasons are for returning to the city... Is he under the effect of its call as Avernus was starting to be?
Oh, and that reminds me. Didn't Avernus believe the story of the Magisters to be a myth? Because he still felt drawn towards the Black City and felt it held the answers to the Taint, which makes me wonder why he felt that way if he didn't believe the story of the Magisters...
Current theory: Those who have lived with the Taint for over a century start sensing the connection to the Black City. It's as good a theory as any.
Speaking of which, Corypheus was asleep for more than a thousand years, and people who sleep wind up in the Fade whether they are aware of it or not, isn't that how it goes? Now, weren't the highest ranking Magisters in Tevinter also Dreamers? So, I assume that if a group of Magisters were all getting together to use most of the Empire's Lyrium, they were probably high ranking Somniari.
If we can take this to be true, Corypheus could have been wandering the Fade for an unprecedented length of time, with power to rival any Demon, and possibly being subjected to the call of the Black City he could never approach the entire time. I'm not sure why he would be so disoriented when he first awoke, but he seemed to snap out of it after possessing either Janeka or Larius. A lot of that could have just been the shock from waking up for the first time in so long, and whatever else may have happened. He seems to know exactly what he's doing after the whole possession incident.
So yeah...
I think the Black City is going to be the true threat looming on the Horizon, and Corypheus is the guy who'll bring it a bit closer to home.