[Legate of Amon Lanc]

That's a thing I believe most people must have noticed when they read the passage you quote. Personally, I thought along the lines you say in your post when first reading it: specially mentioning that not in this age of the world would imply that later it was heard; and knowing the scheme of "Dark Lords of descending power" (Morgoth is defeated and his most powerful servant Sauron is the new Dark Lord, so Sauron is defeated and his most powerful servant WK is the new Dark Lord?) I came to a conclusion that in the course of Arda, it will make sense. However, there are several counterarguments: First, Tolkien says that the Fourth Age and the ages later are only in the dominion of Men and that there will no longer be "mythic", "magical", or "supernatural", or how should I call that, enemies for them (I believe it is somewhere in the Letters - if anyone can provide the quotation, it will be helpful). Consequently, I thought about Dagor Dagorath - in such a moment as the last battle at the end of times, all the requirements would be met (it is another age, and all the enemies return). That's, in my opinion, the occassion to which we can with clean conscience place the event when we hear the WK's cry again.

Another thing speaking against the return of WK without any Sauron to be his superior is the fact that as a Nazgul, he was totally subdued to the One Ring and when it was destroyed, his own Ring lost all its power, and therefore, his spirit departed, having no longer a mortal body nor any will binding him to this world.