G - d's Promises which have not yet been fulfilled

B"H

Sometimes while studying Torah, we obviously don't pay too much attention to every little detail. I particularly came to this conclusion while studying Rambam's "Mishna Torah - Hilchot Melachim u'Milchamot" 11:2.
There the Rambam wrote:

"Similarly, in regard to the cities of refuge, (Deuteronomy 19:8 -9) states:"When G - d will expand your borders …, you must add three more cities". This commandment was never fulfilled. Surely, G - d did not give this command in vain".

As to the prophets, there is no need to cite proofs from the prophetic Books, for all their books are filled with mention of this matter.
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The Rambam's intention is not merely to cite another proof that the Meshiach will come, but, rather, to demonstrate that he will enable the Jews to observe Torah and Mitzvot in a more complete manner.

The cities of refuge were for people who, by mistake, killed a fellow Jew. They had to remain in such a city until the death of the present Cohen HaGadol (High Priest). After he died, they could leave the city without expecting further punishment.

"You must add three more cities". - In the territory of these nations in addition to the six established in Eretz Israel and Trans - Jordan (Hilchot Rotzeach 8:4).
This commandment was never fulfilled. Surely, G - d did not give this command in vain. - Hence, we may assume that it is to be fulfilled in the Messianic age.