Third and fourth are Willas and Garlan Tyrell, the older brothers to Loras and Margery Tyrell. "I didn’t just put them in for hoots and giggles, they have roles to play in the last two books, and they don’t exist in the show," Martin said
Maybe we could get Tyrion involved with them somehow and he could ask them to clarify a position on something, and Tyrion could ask, "What you talking 'bout Willas?"
Yep, right after Willas mispronounces his name as "Tyrone."
As for next season's GoT, there are plenty of storylines that have been left out that can still be visited. The show could be taking a cue from AFFC and ADWD, focusing on some of the plotlines that have been left out:
Riverlands/Freys, and something for Jamie to do now that Dorne has been visited. Stoneheart, and Brienne's fate now that she's confronted Stannis.
Iron Islands/Victarion haven't even been touched. Asha(Yara) hasn't been seen in a while.
Maybe Aegon is introduced and takes his case to Dorne, rather than to Dany. The Dorne/Aegon storyline could set up Dany's return to Westeros.
Take those, sprinkle in Cersei, Arya, Sansa, Bran, Jon, Dany, Tyrion, and some walker stuff, and you have plenty to go on for Season 6.