Over at Tor.com Chris Lough considers the possibility that HBO's Game of Thrones may come to an end before the series of novels on which it is based gets its concluding volume into print. Might, then, the TV series (projected to run about seven seasons, and thus wind things up circa 2017) offer the ending of the saga before the books do?
It certainly does seem plausible that we will not get both of the last two volumes to George R.R. Martin's series within the next four years, given how slowly volumes four and five came out (A Feast for Crows and A Dance of Dragons taking some eleven years to appear after A Storm of Swords). Still, as Lough points out, the series' creators have many alternatives to actually presenting that conclusion, including the series giving only some of the ending; leaving the real conclusion to a film to come out later; or putting off the conclusion for a bit longer by going on hiatus for another season.
All of these strategies have been used before (though given the norms of American TV, that first option, giving only some of the ending, seems most likely). Yet the challenge for the series' writers begins well before the conclusion. A Storm of Swords offered material sufficient for two seasons--but the fourth and fifth books were rather less satisfactory for many readers, because so much of what they contain seems to be of marginal importance to the whole.
Arya and Sansa do continue their journeys--but these are much further removed from the core of the conflict in Westeros than in the first three volumes. The same goes for Tyrion's adventure after his escape from Westeros, while this seems even more the case with Daenerys' time attempting to establish a new order in Mereen in the fifth book. The attention these books devote to established characters now being used as viewpoint figures (Brienne, Samwell), and new characters in new places which had received little direct attention prior to these volumes (the events in the Iron Islands and Dorne) only deepens the impression of this part of the story as looser and lacking in significant events.
Only Cersei's misrule in King's Landing, Stannis' struggle against her and Jon Snow's tenure as Lord-Commander of the Night's Watch remain at the heart of the drama, and even these portions of the books lacked the tightness of their earlier treatments. The fact that Feast and Dance mostly depict events that happened simultaneously (with Crows dealing with only some plot threads, and Dragons picking up others where Storm left off) adds yet another complication.
Next to what came before, it can seem diffuse and anemic, and it ought not to be assumed that viewers of the show will be more forgiving than readers of the books, confronting the writers with a significant challenge if they mean to hold their interest for another three seasons, and making some alterations seem all but inevitable. One is that they will synchronize the events of Feast and Dance (so that, for instance, we will see Cersei's and Tyrion's plots unfolding in the same episodes). Another is that they will compress these events, perhaps by turning them into a single season by dropping anything not absolutely essential to the story's trajectory. (I certainly expect that we will see much less of the Iron Islands and Dorne.) I also think we are likely to see rather more revision of the material that is retained than we have seen in the series to date (throwing in as many surprises as the story can stand to spice things up).
Of course, even after all that, I doubt the results will match the vigor and pace of the first four seasons, but they might be sufficiently strong to hold on to the viewers' loyalty until the revelations of the (hopefully) more eventful The Winds of Winter.
My Posts on Game of Thrones/A Song of Ice and Fire