Game of Thrones would've been very different if George R.R. Martin had stuck to his original plan for his A Song of Ice and Fire book series. The TV show and books present two different versions of the same story, with Game of Thrones massively expediting things in order to get to to the finish line, something Martin himself still hasn't managed.
Game of Thrones started back in 2011, the same year the latest entry in A Song of Ice and Fire, titled A Dance With Dragons, was released. GRRM had hoped to complete at least The Winds of Winter, if not A Dream of Spring, before Game of Thrones finished its run, but alas it wasn't to be. Some eight years later, and after many delays, The Winds of Winter still doesn't have a release date, and Game of Thrones had an ending that might be the same as the books, but with a rather different journey.
However, things could've been even more different for the TV show - if it happened at all - if Martin had gone with his original ideas. He's well-known for shifting things and finding the story as he moves along, but some of his initial plans for the series that became Game of Thrones would've made things unrecognizable.
A Song Of Ice & Fire Would've Just Been A Trilogy
George R.R. Martin first started working on A Game of Thrones, the first book in A Song of Ice and Fire, back in 1991 when, as per the author himself, the vision of a young boy seeing a man beheaded and finding wolf pups in the snow popped into his head. That became the first chapter of the book (after a very cold open), with that young boy Bran Stark. In 1994, he sent a story outline and 200 manuscript pages of the first book to his agent, Kirby McCauley, which detailed his plans for a trilogy. Yes, back then A Song of Ice and Fire wasn't supposed to be seven books, but just three.
The first book was slated to be A Game of Thrones, which was going to document the rivalry between Houses Stark and Lannister, with the Iron Throne as the ultimate prize. So far, so much like what happened in the books and Game of Thrones. Where the overarching story gets different is what comes next. The second book was tentatively titled A Dance With Dragons. Here, while the Starks fight Lannisters, a greater threat emerges from the East, with Daenerys Targaryen and her Dothraki horselords ready to invade Westeros.
In the third and final book, The Winds of Winter, the biggest danger of all, the mysterious Others, would make their play, culminating in an epic climax that brought together all the major characters and plot threads. Martin envisaged each book as being around 700-800 manuscript pages, but when A Game of Thrones went well over 1000 pages, he realized it should be four books, and then eventually six, with two different trilogies encompassing one whole story, that then eventually became seven.
Some Game Of Thrones Characters Would've Been VERY Different
In his original plans, Martin had ideas for a number of the biggest characters, few of which were actually maintained. Some of the core elements didn't change, such as Ned Stark being killed at the order of the despicable Joffrey Baratheon, or Jon Snow becoming a brother of the Night's Watch. But many of the characters were different, some to the point where it's hard to imagine the events once envisaged ever playing out.
Jaime Lannister would've been more of an outright villain, rather than the complex character Game of Thrones fans hated then loved, and after Tyrion killed Joffrey it'd be Jaime who took the Iron Throne, killing all before him to ensure it was his, and blaming those murders on his brother. Sansa, meanwhile, would've ultimately sided with Joffrey and the Lannisters against her own family, and was set to give Joffrey a child too, although she'd eventually come to regret these things. Things were to still go horribly wrong for the Starks, of course: Robb was to die in the field of battle, although not before maiming Joffrey, and Tyrion was to lay siege to Winterfell, burning the castle down. Catelyn and her remaining children were to flee north, first to the Wall, and then beyond it, where they'd find refuge with Mance Rayder - but Catelyn would die at the hands of the Others.
As if that weren't all different enough to the events of A Song of Ice and Fire and Game of Thrones, there was also going to be a love triangle developing between Arya Stark, Jon Snow, and Tyrion Lannister, with Arya realizing she had fallen for her half-brother (still secretly her cousin), while Tyrion would come to find he was in love with Arya, leading to a rivalry between him and Jon that would play out in the second book.
GRRM Later Planned A Time Jump After The Third Book
Having already made a number of changes to his initial plan, and seen his story massively expand, George R.R. Martin planned to include a time jump after the third book in the A Song of Ice and Fire series, A Storm of Swords. With the book set to focus primarily on Daenerys' return to Westeros, Martin decided that a five-year time jump would help move things along, allowing the younger characters of the story to age up and for the dragons to grow larger. This was considered necessary since further installments would have things like Arya becoming an assassin, Daenerys a ruler, and Sansa a skilled player of the game, for which Martin didn't want them to still be kids or teenagers.
The idea, then, was to cover the events that happened during those five years via flashbacks and retrospection, but eventually Martin decided that didn't satisfyingly cover things. At the same time, it was impossible to leave out those events, because then there'd be a huge gap in the story, where we'd just have to assume characters like Stannis and Jon Snow sat and did nothing for half a decade. That's when A Song of Ice and Fire expanded again to include the seventh book, with A Feast for Crows initially designed to bridge the gap between A Storm of Swords and A Dance With Dragons, before instead becoming part of the same concurrent story as the latter, just with many major characters absent.
Why GRRM's A Song Of Ice And Fire Plans Changed
The biggest reason that George R.R. Martin's plans changed is that, well, Martin is not all that great at planning. He's admitted himself that he is a "gardener" when it comes to writing, which means that he likes to see where the characters take him, rather than planning out every tiny detail. That means as he started writing more of A Game of Thrones and its subsequent novels, the characters took him in different places: it became clear that an Arya/Jon/Tyrion love story wouldn't work, for example; Jaime became more complex; Catelyn and Robb's deaths became the Red Wedding.
That goes with the other major reason why Martin's plans changed, and why he's struggling to write The Winds of Winter. This started off as a relatively tight story: there was a clear structure, with an obvious beginning, middle, and end. The more he's written, though, the bigger the story has gotten. There are so many characters, locations, plots, and sub-plots, that it would've been impossible to tell it all in three books, and it might yet prove impossible to tell in seven. That's partly why Game of Thrones' ending happened so quickly, because they had to excise a lot of material in order to actually get to the finish line. George R.R. Martin will hopefully finish A Song of Ice and Fire, but it's definitely going to look quite different to when he started.