The Legend of Zelda: Breath of the Wild takes place in the iconic Kingdom of Hyrule, but hints at the presence of a world far beyond that which the game allows players to explore. As an open-world game, Breath of the Wild has a limit to where players can venture to, primarily marked by invisible barriers or impassable terrain. However, a vast horizon points to canyons, forests, and oceans that stretch far beyond the eye can see. If Hyrule was able to survive the Great Calamity, it's not unreasonable to assume other locations and races did as well.
100 years prior to the events of Breath of the Wild, a huge disaster wrecked Hyrule. Ganon, transformed into a monstrous beast of pure Malice, brought a massive calamity to the lands. Massive battles occurred which left Hyrule in ruins. Ultimately, Calamity Ganon was sealed away by Zelda, who marched alone to Hyrule Castle and used her powers to trap both Ganon and herself away, ending the Great Calamity. The game picks up when Link awakens to find the remains of Hyrule. Given that some of the kingdom survived, alongside the Hylians and 8 other races, it's possible some migrated to Hyrule after the events that had occurred.
Breath of the Wild both directly and indirectly implies the possibility of travelers coming to Hyrule from beyond what players can access. Off the coast of the Necluda Sea is Lurelin Village. This small fishing village is one of the only to have survived the onslaught of Calamity Ganon. With a little exploration, players can find boats waiting on the sands, some loaded with fishing equipment. A few of the central buildings in Lurelin also resemble large ships. This confirms that people were not bound to Hyrule, allowing them to leave and return at any time. Perhaps outsiders used the ocean as a means of transportation to get to Hyrule themselves. The Akkala Citadel confirms this did occur. In "Creating A Champion", the citadel's primary function was to repel invaders from across the sea and keep them from entering The Legend of Zelda's Central Hyrule. If people are entering Hyrule lands, they must have come from beyond the explorable territory.
The World Outside of BotW's Hyrule Kingdom
Given the prevalence of oceanic travel in BotW, there's a likelihood that the Necluda Sea is host to a variety of island clusters. Ocean transportation would not be a new feature to the Legend of Zelda series given the existence of Wind Waker. Wind Waker features 49 unique islands, each of which the player can explore. While the game takes place in the Great Sea, a location that is nonexistent in BotW, a similar premise could be established. Given that island locations in The Legend of Zelda series like Outset Island and Forest Haven could sustain life, perhaps the Necluda Sea is filled with its own isolated groups.
The Oracle series (Oracle of Seasons and Oracle of Ages) in the Legend of Zelda franchise feature two nations that are known to be completely outside of Hyrule. Labrynna and Holodrum are sister lands known for a diverse ecosystem which encompasses vast mountains, ruins, and forests. Given that it's never really established where these lands are located in relation to Hyrule, it's quite possible that BotW could be set quite close to them. Given the similarities between their geographical makeup and that of the Kingdom of Hyrule, these locations might just be neighbors.
There is definitive cause to believe that there is a lot more to The Legend of Zelda: Breath of the Wild than what players can currently explore. What these possible locations may be is very much up to speculation. With Breath of the Wild 2's trailer depicting Link and Zelda traversing previously unseen underground caverns, it's possible the next game in the franchise will reveal exactly what lies beyond the edges of Hyrule.
Source: Zelda Wiki