Like TDKR, a battle in the Fade as Corypheus makes for the Seat of the Maker. It would give us a chance to see how corrupt the City truly is. Oh no doubt it's black as sin, but we could bask in the decadence and corruption that has so stained what many believe to be the hallowed halls of the Creator himself.
Definitely also the mind game idea. Corypheus as a boss was lackluster. He has no special talents, despite being a goddamn Tevinter Magister with access to the Blight. Have him bring his wroth down upon you in all its glory. Firestorm, Winter's grasp, Chain lightning, etc. Have him play by the same rules we play as. Give him the skills we have.
Not a goddamn swipe of his hand and a pitiful red lyrium mine.
if the Dragon were to remain a boss fight, have it also fight in a better fashion. As it was, it happened to be a very tedious chore to deplete its health because it was a dumb **** who kept doing the same abilities. My final team managed to deal with it intelligently, but there was no fun in the fight to me (unlike, as I have often referenced, FINAL ****** FANTASY XII). Dragons in the game were pitiful. Oh, the Fereldan Frostback was challenging enough, but once you beat that one the rest were easy to kill.
Apex predators my ass...
But yeah, a solo fight with him would be rather kick-ass, especially if he -- again -- had a more diverse attack paradigm.
And perhaps have him be a sequential boss. That is to say, we face Corypheus in one aspect, and then another. But then, I'm a sucker for boss fights like that. They truly can give off an epic sense to you. And when you're going up against someone who wants to be a god... well... I think taking a cue from how FMA:B did their idea isn't a bad idea.
(and I mean for the whole arc. A masterwork gambit on Father's part, even if he was outmaneuvered). Corypheus' entire plan is... great in concept -- and we're told he is a threat -- but in terms of how Bioware decided to go about giving us the scope of his actions... not so much.
ALSO, instead of the Final battle just being this "You immediately arrive" stuff, we cleave our way through a host of powerful Demons -- who would ideally also fight in a better capacity, as I found them rather dull as well -- before finally facing him.
And yeah, more dialogue with him would be nice. Perhaps have it during the battle itself. Be nice to see if we could rattle him a bit.