Slim,
A person has zero prospect of surviving an event they won't live to see. Therefore it would have been impossible to be a "prospective" member of the great crowd in the year 1492, for example, because here we are 500+ years later.
What you're suggesting, if I'm understanding you correctly, is the removal of that chronological element, which would create a third distinct group of Christians who do not actually fall into either of the two categories that JW's believe are described at Revelation 7 & 14.
I can't find anything like that in their literature. They've taken in recent years to describing the earthly hope in equivocal terms, that distract from what they believe Revelation 7 is specifically describing, but sloppy language is not much to base a conclusion on.
The existence of other sheep apart from the great crowd during the Christian era would create a bunch of other doctrinal problems as well, especially since JW's believe that with a few rare exceptions, everyone gets resurrected anyway.