Media speculation from when the game was first announced was that Dragon Quest Builders would be a Dragon Quest version of Minecraft. Early screenshots show the player building a camp out of blocks; the camp gains levels based on how much it has been built up. Materials that are gatherable by the player include "rock, wood, fruit, plantlife, slime ooze, and more". Monsters such as Slime and Dragon in original Dragon Quest will appear in the game. The player has a health bar, and early screenshots show a food meter.
The game is set in Alefgard, the world of the original Dragon Quest. The game is based on a parallel ending of Dragon Quest: before the final battle, the hero accepted the evil Dragonlord's suggestion - each rules a half of the world. But that's indeed a trap, and the hero was eliminated, then the world was dominated by monsters. With time passing by, a new hero (the player character) appears to revive the desolate world. Players gather "material" all over the world in order to rebuild the land of Alefgard from scratch.
Source: Wikipedia, "Dragon Quest Builders", available under the CC-BY-SA License.