Refuge from death
ואם-ירחיב יקוק אלקיך את-גבלך כאשר נשבע לאבתיך ונתן לך את-כל-הארץ אשר דבר לתת לאבתיך: …ויספת לך עוד שלש ערים על השלש האלה
If Hashem will expand your borders, as He swore to your forefathers, He will give to you the land that he spoke to give to your forefathers…and He will increase for you three more cities, in addition to these three
The Torah has an interesting concept known as the Arei Milkat, the Cities of Refuge. If someone were to accidentally murder another, the Torah commands this person be exiled to the Arei Miklat. They serve simultaneously as an atonement for the person’s lack of precaution, and as a safe refuge from any relatives that may want to avenge their deceased. The Torah mandates three cities on the east side of the Jordan River, and three on the west. However, a verse in our parsha speaks of the future, the Messianic days. In those days, the land of Israel will expand in size. The Torah tells us that with this added territory will come three new cities of refuge. A question that many ask on this is that in the future, no nation shall lift up sword against nation. In fact, death will be abolished. If so, what is the need for three new cities of refuge? The original ones will prove obsolete, as there won’t be any more accidental murders, so why add more?
There seems to be a debate as to the exact way in which the final Messianic redemption will unfold. Some say that as the process unfolds and the land of Israel is expanded, there will still be a natural order of things. The Cities of Refuge will therefore still be necessary. Others say that while wars will cease, initially death will still be possible. People may still accidentally murder others. They will still need a place of refuge. And yet others say that those who killed in the days preceding the Messiah will still need to achieve atonement in the cities of refuge once he comes. These extra cities then will accommodate all the accidental murderers throughout the exile, even though death will be abolished.
Some say that the futuristic existence of no death will only exist for the Jewish people. New converts to the faith after the coming of the Messiah will still be subject to nature, and could still be liable to exile. Others don’t limit this to converts, but say that non-Jews could still be within the confines of the natural order. If they accidentally killed each other, they would also be subject to the Arei Miklat.
Our Sages tell us that the Messiah can come in two ways: in his designated time, or we can merit to bring him early. If our good deeds aren’t enough to bring the Messiah early, then when he comes it will be a miraculous event. For, in essence, we don’t deserve his arrival. Since his arrival will be miraculous, it is in this state of being where world will function supernaturally, where there will be no war or death. However, if our merits allow the Messiah to come early, then the world will function normally. In the situation where the Messiah comes early, that is when there will be a need for new Arei Miklat.
This final approach is why the verse is stated as a hypothetical: “If Hashem were to expand your borders”. Why is this something that G-d appears uncertain about? The reason is Hashem is telling us that it’s not set in stone. It’s dependent on our actions. If we fail to bring the Messiah early, he will have to come in a miraculous fashion. We will then have no need for Arei Miklat. However, if Hashem expands your borders because the Messiah managed to come early, since the world will continue in a natural way, there will still be accidental murders. They will still need a place to seek refuge.
 Based on a shiur given by Rav Daniel Glastein, found on https://www.torahanytime.com/#/lectures?v=159408 I added some sources to what he provided
 Deuteronomy 19:8,9
 Makkos 2b; cf. Makkos 11b (however, see Tosafos ad. loc. s.v. מידי גלות)
 Numbers 35:25
 Rashi, quoting Sifrei Devarim § 184
 Isaiah 2:4
 Ibid 25:8
 Shela Hakadosh in his Shenei Luchos HaBris Hakdamas Toldos HaAdam Beis Dovid
 Aruch LaNer to Makkos 9b s.v. בגלעד
 Seemingly just for atonement, as you would think that people in the future won’t feel a need to seek revenge for their relatives’ untimely demise. However, see The Lubavitcher Rebbe’s approach to this in Likkutei Sichos XXIV p. 107ff
 Yedei Moshe to Devarim Rabbah 4:30; Lubavitcher Rebbe loc. cit., citing the Alshich to Deuteronomy loc. cit.; Chida in his Penei Dovid Shoftim § 11, in the name of the Sheivet Mussar
 Rav Daniel Glatstein also cited a reason from the Minchas Chinuch § 520, that it’s simply a gezeiras hakasuv, a decree of the Writ, for there is no reason to build more. Rav Glatstein added a reason that in the future everyone will miraculously become righteous, so we won’t need cities of refuge, but we aren’t supposed to rely on miracles. He didn’t cite by name who he was quoting
 Chasam Sofer in his Toras Moshe I to Deuteronomy loc. cit.; Maskil L’Dovid ad. loc. The Chasam Sofer Al HaTorah ad. loc. gives another explanation, that Chazal say that Ba’alei Teshuva are on a higher level than those who never sinned (Berachos 34b). As such, those that never sinned will need the Arei Milkat, unlike the Ba’alei Teshuva at the end of days
 Rav Chaim Kanievsky in his Nachal Eisan 1:1:6, and Ta’amah D’Kra ad. loc.
 גר תושב אינו גולה אלא על ידי גר תושב (Makkos 8b)
 Sanhedrin 98a, based on Isaiah 60:22
 Although, one could have thought the opposite would be true…
 Meshech Chochmah ad. loc. His source seems to be the Arizal in his Likkutei Torah parshas Shoftim, brought by the Shela HaKadosh and Maskil L’Dovid loc. cit., although the Arizal’s approach is unsurprisingly more kabbalistic
 Ibid. See also Alshich loc. cit., who says this final part as well