On Twitter late last year, I saw what was probably the single best idea that would have greatly improved the last few episodes, and could probably have been accomplished with a single scene (and maybe still could for a future "definitive edition").
The idea is that at some point prior to the last two episodes, Danerys receives word that Mereen has fallen, and that the slavers have retaken it. Her greatest achievement in "breaking the wheel" is now gone, and her designs to be the next Aegon are no longer nearly as assured.
Building on this idea, the scene could play out with her completely losing it and threatening to burn Mereen to the ground, her advisors only barely talking her out of flying across the Narrow Sea right then and there.
Mereen's destruction is averted, but the seed has been planted in the viewer's mind: Danerys has much less to lose now, and she's willing to rain fire on a populace with enough provocation. Now when the other shoe drops and she does this on King's Landing, the turn has enough narrative foundation to feel plausible and earned.
You say that the problem is the execution. I think it goes deeper. It's impossible to execute. GRRM knew this about many plot points and that's why he changed his mind about many things by the time he had to expand on those specific plot points.
> Bran ending up on the throne kinda makes sense
If he ever gets to the end, I'm certain he'll see that this doesn't make any sense.
Or at least he won't write the same bullshit the show gave us: "he deserves the throne because he has the best story". I still want to throw the TV at the wall when I think about that.
I think you are spot on about ASOIAF, but I don't think this is the reason GoT ended badly. Books and TV Show started diverting much earlier, and the show had substantially fewer storylines, which is a good thing in this context I think. I also suspect that the ending we got is based on notes GRRM left. Daenerys turning evil is heavily foreshadowed, Bran ending up on the throne kinda makes sense, and so does Jamie not cashing in his redemption after all. WW not being the final arc mirrors LotR, and GRRM has repeatedly stated that he loved that way of storytelling. The first book is called Game of Thrones, the ending is about that as well.
The problem in my opinion was just terrible execution: It was all really rushed and with terrible dialogue, so none of the story elements were developed in detail, and felt artificial and wooden.