A prophetic vision outlines the future expansion of Jerusalem, transforming outer areas and even places associated with death and waste into an integral part of the holy city. In the future, vast new territories will be brought within the protective walls of Jerusalem.
These new borders will encompass areas that once held a dark history, such as the valley of the dead. The primary approach among commentators is that this refers to the valley where the massive army of the Assyrian king Sennacherib fell [רש״י, מצודת דוד]. The expanded city will also take in the grounds outside the original walls where the ashes from the altar sacrifices were traditionally discarded [מצודת ציון, מצודת דוד, ביאור שטיינזלץ]. Alternatively, this area might also be associated with the Assyrian camp, pointing to the once strong and healthy soldiers who met their end in that very spot [רד״ק]. Beyond these valleys, the new boundaries will sweep across the neighboring fields [רש״י, מצודת ציון], along with the aqueducts that supplied them with water [רד״ק].
Stretching eastward, the city limits will reach all the way to the Kidron Valley and the corner of the Horse Gate. Historically, this gate served as the main entrance for horses into the city [מצודת דוד]. It was also the starting point where the king and his riders would exit onto a flat road specifically designed for horse racing [רד״ק].
All of this newly incorporated eastern territory will be designated as holy to God, acquiring the exact same sacred status as the rest of Jerusalem [מצודת דוד]. It is even possible that portions of this new land will be absorbed directly into the Temple complex, expanding its sacred borders [רד״ק]. Ultimately, the prophecy promises complete permanence, ensuring that this newly built city will never be uprooted, ruined, or broken down [מצודת ציון]. Unlike the First and Second Temples which eventually fell, the future Jerusalem will never again be conquered by an enemy, standing secure and completely safe forever [מצודת דוד, רד״ק].