I've been advocating abandoning the foolishness of irrevocably dating the Eugenics War in the 1990s for a while. It didn't even make sense in the late 1960s. For Khan to have been a ruler of a quarter of Earth in 1992 or whatever it was, "Space Seed" was asking us to believe that there was a massive genetic engineering or selective breeding program being undertaken somewhere in the East even as the episode premiered.
I don't really want to see Khan or the Augments in Star Trek 12, but if I did, I would be pleased to see them from at the earliest the 2050s.
This is the conclusion I eventually came to as well. In writing my biography of Khan, I had him born in 2001, rise to power in 2042, and be overthrown in 2046. Read about it here: