What is the meaning of "Ki Im Sisu"?
Radak: Kel tells Yisrael to rejoice forever due to the great good. He will not tell them another matter, i.e. anything bad, only this good matter.
Malbim: Sisah is doing actions to reveal the Simchah, e.g. with drums, lyres and wearing Yom Tov clothing.
Will Hashem make a new creation?
Radak: It will be new. Something new is called Beri'ah, like "Bereishis Bara Elokim..." (Bereishis 1:1), "v'Im Beri'ah Yivra Hashem" (Bamidbar 16:30).
Malbim: Gilah is in the heart. It is sudden Simchah due to a new tiding. It cannot be constant. Even constant Simchah is not possible based on man's nature. If the good is constant, he gets used to it, and does not rejoice in it. However, then Simchah and Gil will be constant. In this world, Gil is only for something that he or someone else initially lacked. There is no Gil for having eyes and a mouth. Then there will be no lack, only existence. Even so, there will be Gil - this is a new creation, impossible in this world.
Why does it say that He will create "Yerushalayim Gilah [v'Amah Masos]"?
Radak: Its name will be Gilah, and its nation's name will be Masos. Even though there will be Shlomo in the entire world, He mentions Yerushalayim, for it will be primary. Shalom will go out from it to the world, and also Torah and the good path [will go out from there], which cause the Shalom - "v'Diber Shalom la'Goyim" (Zecharyah 9:10), "v'Halchu Amim Rabim v'Amru Lechu v'Na'aleh El Har Hashem El Beis Elokei Yakov v'Yorenu mi'Derachav v'Nelechah b'Orchosav Ki mi'Tziyon Tetzei Sorah u'Dvar Hashem mi'Yerushalayim" (above, 2:3). The great good, Shalom and longevity will be in Yerushalayim and Eretz Yisrael - "Nachon Yihyeh Har Beis Hashem b'Rosh he'Harim v'Nisa mi'Geva'os" (2:2).
Malbim: Yerushalayim itself will be constant Gilah via the Shechinah and Divine light that will be seen there. The external signs used to reveal this Gil will be the nation itself. It is as if via the nation, their success, and multitude, the Gil will be revealed, like one who reveals his Simchah via wearing Yom Tov clothing,
