The dragons should have immediately went for the Red Keep. Nobody has any kind of air force, only Daenerys. She basically has flying nukes. Scorpions? First of all, as we all know it, that concept is stupid as hell, but the dragons can fly high enough anyway. Or do it as a sneak attack.
If the story needs Cersei to hold King's Landing, she could have used her own people as hostages - like radical islamic states -, since Deny cares about the innocent, and Cersei doesn't (especially after blowing up the Sept.) It would have made Deny's moral descent more believable, since now there's enough reason for her to kill those innocents, as she becoming more and more impatient to get the throne, and snapping, slowly giving in to the lust of power. Maybe this could have also been the reason Jaime becomes disillusioned with Cersei, and left. (Well, Tomen's death and blowing up the Sept should have been enough reason, but whatever.)
After that they could have shown a proper change of Deny during the war against the White Walkers. I don't know. So many story possibilites were thrown out mindlessly, it still hurts.
(But of course the writers had to take the classic action movie route of aimlessly extending the primitve plot, and defeating the enemies at the end. Because that's what the viewer wants, right? Not the rational, politically nuanced nonsense that was Game of Thrones. Not that they have done that right: if that's what you want, then why wouldn't you let Jon Snow confronting the Night King? GRRM's surprises are always logical, that's why they are good.)