I don't own anything except my ideas, although this little possibility is quite believable.
The Winter War ended in total devastation, both Hueco Mundo and Soul Society were utterly destroyed, along with every supernatural faction, light, dark, and neutral alike.
Earth's landscape was Shattered by the side effects of the destruction of both realms, only a fraction of the population survived, much knowledge was lost to the damage, humanity was reduced to primitive technology.
The spiritual powers of Soul Society and Hueco Mundo returned to the world of the living, the light powers of Soul Society returned to humanity, while the hate and rage of Hueco Mundo had to go somewhere.
These dark powers Twisted many wild animals into a cruel parody of life, visible to the living unlike before their bodies blackened without their souls, they donned the masks of Hollows, and their hunger for souls drove them to hunt humanity.
Humanity named their new world Remnant, and defended itself from this new soulless threat, calling them 'Grimm'.
Thousands of years later Hunters and Huntresses roam the land, and humanity are on the cusp of finding out their origins, while war may be on the horizon. Who knows what the outcome will be...