As much as I would like to get some Vanguards, VC10, Convair 880/990, DH Comet etc in CS quality, I guess the market is limited and most likely not to happen. The reason being, that even in their heyday in the 60'es they were rare birds, and hence you may have to be 50+ to have any relation to them. My guess is, that any developer would try to get also the younger folks into the hobby.
It's a different story with the 707, DC8 and early 747's of the same era, as they were built in large quantities and still could be seen as freighters, Awacs and tankers until very recently.
By the way, there are Comets and Viscounts available today for FSX. But honestly, the freeware versions for FS9 are vastly superior IMO, the very reason I still keep the older sim on one of my drives.

Hence, my vote go for DC8, DC10 and early 747.
Best regards,
Anders