i mean it's nothing short of a miracle that Corypheus doesn't get himself killed in one of his idiotic plans,
Actually, he did get himself killed between the two elvhen statues that guarded the Temple of Mythal.
In all honesty, a lot of things in Inquisition were sloppily done.
For example, if Samson or Calpernia had simply closed the doors to the Temple of Mythal, then the Inquisitor and company would have been red lyrium roasted by the dragon.
Same with Haven. If the Herald just buried your army with a trebuchet, don't you think you should... I don't know... have your dragon take out all the trebuchets? Or have your remaining forces do it? And if your dragon just got killed, thus leaving you vulnerable (in the final battle), then wouldn't it be a good idea to run away and hide until the dragon respawns?
As for what they could have done with Corypheus, yes they could have done so much more. Until the last part of the game, it was suspected that he was immortal, and indeed unless you knew how to kill him, he was immortal. This was the perfect setting in which to have him as a reoccurring enemy that you kill over and over just to see him resurrect soon after within a nearby Warden, or red templar, or darkspawn, or even a tainted wolf. This would add to suspense as well as provide numerous scenes in which you can show Corypheus being killed in a variety of ways. Then each time you face him he could be stronger, or take on different forms.
For example, we learn from Bianca that Cory was in the form of Larius or Jenaka when he met with her about the red lyrium. So why couldn't we face Corypheus in the form of a Grey Warden warrior, mage, rogue? Adamant was a perfect opportunity for this since Hawke was there too. We could have had a midway point where Corypheus shows up in a Warden host and then attacks with a room full of Wardens, some of which he controls, others he just influences to where they won't strike at him. Each time you kill Corypheus he resurrects in another Grey Warden that is cowering off to the side. You then have the choice to run around to each vulnerable Warden and kill them yourself to cut down on how many times Corypheus respawns, OR you can let him respawn in each one, OR you can try to focus your energy on destroying a part of the room that will allow those Wardens to escape. The moral choice being the most difficult, of course. The downside would be that this could conflict with Blackwall's personal story, but he could either be blocked from that mission or he could just "explain it all later at Skyhold".
Having Corypheus suddenly resurrect in one of Skyhold's Wardens would also give opportunity for a fierce battle that would involve numerous familiar NPCs. Of course, this battle would start once a Grey Warden makes an attempt on your life in the main hall. Pandemonium would then ensue. This is only if you made the Wardens your allies and allowed some of them to visit Skyhold. I would have also liked to have seen the red lyrium dragon attack Skyhold at least once. It could have flown to the undercroft and the prison cells as part of the battle, where you and the others would have to force it out of there. Dagna would have found such an event most entertaining I'm sure.
The darkspawn should have been utilized by Corypheus, I think. And there should have been a mission to stop Cory from possibly uncovering and corrupting one of the two remaining old gods.
I think BioWare got hung up on trying to present the Corypheus story and the Mythal story as epic as they could, and ended up doing neither one particularly well.