Rashi: It is the valley in which the Pegarim (corpses) of Sancheriv's camp fell.

Radak citing Targum Yonasan: It is the plain in which the Pegarim of Machaneh Ashur fell.

Rashi: It is where they spilled the ashes outside of Yerushalayim. They will add to Yerushalayim, and all this will be within its walls. This Nevu'ah is for the future.

Radak: This is like "Dashen v'Shamen" (Yeshayah 30:23). I.e. Machaneh Ashur, which was Deshen (well fed and fresh) fell there.

Targum Yonasan: The gate of the king's horses. Radak - i.e. the gate from which the king leaves with his cavalry. There was a flat place there for the horses to run; its length was a Ris, i.e. 70 Kanim. Each Kaneh is six Amos and a Zeres (small finger).

Rashi: They are flat fields.

Radak: They are many fields close to the city.

Targum Yonasan: Arisa. Radak - this is like 'Arisa d'Dalai' (of people who draw water), i.e. an irrigation channel from which they irrigate fields.

Radak #1: This is like "Kodesh Yisrael la'Shem" (2:3).

Radak #2: Part of this addition will be in the Mikdash, for also it will expand, like it says in Yechezkel's Nevu'ah. It will not be destroyed like Bayis Rishon and Bayis Sheni were.