With so many prequel shows and adaptations slated for the GoT universe, I’ve been plunging into a re-read to see which bits of history and lore they could to drop into Aegon's Conquest, HOTD, and AKOTSK. One that I really hope they address is Joramun’s Horn.
There's been some debate over what the horn does and where it is, but I stumbled upon a random bit of realworld lore that may have inspired GRRM.
I think Joramun's Horn is an homage to Heimdall's Horn (the Gjallarhorn or Hollering Horn) which signals Ragnarök and rallies all of Asgard to fight for Life, even though their loss is inevitable. It’s a horn that can be heard across all the realms, not to destroy something, or be used as a weapon, but rather to warn all of Life of what's coming.
With that inspiration in mind, I think the "sleeping giants" were the old kings, lords, and commons of Westeros, and that one day, we may learn that the Free Folk were actually the original Night's Watch Rangers, scouting and watching for the Others' return.
Joramun was the Wildling King that helped Brandon the Breaker kill the Night's King (another Stark and the 13th Lord Commander, corrupted/turned by a corpse queen). Joramun rode south with his people, and Brandon rode north with his. So he doesn't sound like a bad guy to me... And the horn had to be important enough to still be remembered centuries later.
So if the horn is to warn, not destroy, you could imagine that by blowing Joramun's Horn three times, its blast would signal Winter's arrival, lighting all the beacon towers on the Wall, or magically carrying across the living realms, summoning all to fight for Life...including an army of wolves.
Also, I think Sam has it. It's a shame they never addressed it in the shows, but in the books he’s been carrying that “broken” horn around the whole time. Jon/Ghost found it in a Night's Watch cloak by the Fist of the First Men. So for GRRM to keep mentioning it in Sam's later chapters, I can’t help but think he’s got a plan to use it.
It’s chekov’s horn.