Week after week, House of the Dragon continues to expand on the lore of Fire & Blood, as the broadened text of histories contains a scarce amount of detail compared to the series. By fleshing out lesser characters and filling in small details that a history text might not know, the show is essentially doing the opposite of what was required for Game of Thrones.
While House of the Dragon constantly has room to expand, Game of Thrones was forced to contract. By Season 5, the books had grown too vast for what the show could accomplish without requiring several more seasons, and the decision was made to leave out a number of plot lines. Left with a rushed ending that felt entirely dissimilar to the complex and harsh world fans had grown accustomed to, it's safe to assume the missing pieces were in the left-out plotlines.
Lady Stoneheart
The Red Wedding was one of the most shocking and heartbreaking moments in the entirety of Game of Thrones, resulting in the deaths of Catelyn and Robb Stark. But death isn't so final for Catelyn as her body is retrieved from a river and resurrected by Thoros of Myr and Beric Dondarrion, who gives his life for hers. She takes on leadership of the outlaw brotherhood under the name Lady Stoneheart.
Rumors spread of the hooded woman hanging Lannisters and Freys throughout the Riverlands in A Feast For Crows. The plot could have tied in well with the Siege of Riverrun, and Brienne's story in the series, as she is the one who encounters Lady Stoneheart in the book. The storyline allows for a more fulfilling tale of vengeance for the Red Wedding, and adds a layer of intrigue and world-building that later seasons lacked.
The Fake Arya Stark
While Arya's story in Braavos has many similarities to that of the show, Sansa's marriage to Ramsay Bolton never occurs. Instead, Sansa's childhood friend Jeyne Poole is the one who is married to Ramsay, pretending to be Arya Stark. It is implied that the Boltons know she isn't really Arya, but use her name to lay claim to the North as they did with Sansa in the show.
However, Jon Snow doesn't know and receives word of her capture in a letter from Ramsay, known as "The Pink Letter." With Jeyne following many of Sansa's show storylines, even escaping Winterfell with Theon, Sansa seems to be embarking on an entirely different plot with Littlefinger and the Lords of the Vale. The plot line in the books more clearly defines Littlefinger's character, as he would never marry off Sansa. It also would allow more room for whatever he is stirring in The Vale, making them more of a player in the final seasons.
Reclaiming The Crown's Lands
As the show compacted, details of the logistics of Westeros began to get foggy. Very little was mentioned of the lesser noble houses and their lands toward the end, with the show even making a self-mocking joke about what might be occurring in the Stormlands. The book is careful to address all of this.
With The War of Five Kings over, Cersei seeks to reclaim the lands lost in the war, and Ser Loras Tyrell is sent to reclaim Dragonstone. Jaime does siege Riverrun in the show, but the book variation results in the Blackfish escaping to fight another day. With a common show criticism being dropped off plot lines, the reclaiming of important castles ties up many loose ends.
Mance Rayder Lives
Burnt alive in Season 5 for refusing to bend the knee to Stannis Baratheon, Mance Rayder's arc abruptly ends. In A Dance With Dragons, Mance's story still has a number of complexities. Melisandre has used magic to fake his burning at the hands of Stannis and sends the real Mance, with the help of Jon, to rescue Arya from Winterfell.
Mance does this as he believes Jon holds his baby captive. However, Jon had actually swapped his baby out with Gilly's in order to protect it from being burnt alive by Melisandre. Many have also theorized that Mance is responsible for sending The Pink Letter. Mance's presence allows for another source of conflict in the North, rather than direct conflict between good and evil.
Cersei's Prophecy
The prophecy given by Maggy the Frog is revealed at the start of Season 5 in a Cersei flashback, displaying her disdain for Tyrion after the murder of their father. While the timing seems to suggest the little brother said to murder her will be Tyrion, many have theorized that it will actually be Jaime. This prophecy is one of Game of Thrones' biggest mysteries and has been the subject of a ton of theory crafting.
Frankly, either of these options would be a more interesting development than her being crushed by rocks. The growing tension between her and Jaime in the books still have readers hoping the plotline will end differently than how it did in the show.
Littlefinger's Master Plan
Littlefinger is the perfect example of Game of Thrones wrapping up a complex character and plot line in an unfortunately simple way. His Season 7 arc scheme and execution reduced him to a plot device in Sansa's character growth when the six seasons prior had built him up to be a character with his own plans and intentions.
Many have theorized on the exact nature of Littlefinger's plan. His actions in having Jon Arryn poisoned kick off the events of the series, so it's very likely that he's more than just an opportunist waiting for his next moment. Littlefinger has tons of sneaky plans in the show, all of which seem more calculated than his plot in Season 7.
The Greyjoy Uncles
Euron Greyjoy is often used as a symbol of the final seasons of the show, with the character being reduced to a plot device for villainy. In A Feast For Crows, Euron becomes King of the Iron Islands following his supposed murder of Balon. But Euron is not the only opposition to Asha (Yara in the show) in the book.
Yara's other uncle, Victarion, also arrives, and it is his refusal to cooperate with Asha that results in Euron being crowned. Euron's intentions are similar to the show, as he plans to use Daenerys' dragons to conquer Westeros himself by the use of a dragon horn he has acquired in his travels. The addition of multiple Greyjoy characters adds a layer of complexity to the Iron Islands and the Kings moot, and each of them is more self-motivated for power than the show's adaptation.
Daenerys' Prophecy
Quaithe is one of the most forgettable characters in the show by the end, which is a shame given how important she was made to seem. In the books, Quaithe carries on in Daenerys' dreams throughout A Storm of Swords and A Dance with Dragons, offering vague warnings and prophetic speech.
Quaithe's intentions are still unclear, but many have theorized that the prophecies tie in with that which she saw in the House of the Undying, or that of Azor Ahai. Both of these prophecies linger on in the books, with far more questions than answers. Daenerys' storyline was one of the show's greatest mistakes, and why Redditors consider Game of Thrones to be one of the shows that aren't worth finishing. Better execution of these prophecies could've changed that.
Conflict In Dorne
Before Season 5 of Game of Thrones, many fans were ecstatic about the plot in Dorne and later saddened to find much of it had been reduced. Quentyn and Arianne Martell, the two central characters in the story, were not set to appear at all.
Doran Martell was one of Game of Thrones' most underutilized characters, as he was simplified as being a weak leader looking to form safe alliances. In A Dance with Dragons, Doran sends his son Quentyn to form an alliance with Daenerys, while Arianne Martell plots to plant Myrcella Baratheon on the Iron Throne. The Dorne plotline adds another player to the mix that is scheming with plans for the Iron Throne. Many removed book plotlines could've spread the final few seasons out longer with more detail, allowing for more satisfying and developed endings.
The Real Aegon Targaryen
Many book readers were appalled at the reveal of Jon Snow being "Aegon Targaryen," when a character of this name already exists in the book. Or so it would seem. Aegon Targaryen, aka Young Griff, is met by Tyrion in A Dance with Dragons.
Young Griff is accompanied by a sell sword revealed to be Lord Jon Connington. The two intend to sweep into Westeros with the power of the Golden Company and take land before the Lannisters have a chance to react. This complication would have provided a third-party issue for both Cersei and Daenerys in the show's final stretch.