Now we have an unrecognized rogue state that in 2006, which will have a GDP PPP of approximately 1.01 trillion USD and a population of about 30.4 million people.
But how would it happen? The UK tried to extend the 99 year lease in 1979, 8 years after recognition of the Official China switched from the Republic of China to the People's Republic of China. The major negotiations would have to occur before 1971, so that the Republic of China would be the legitimate "lessor".
At the same time the major pressure that was there to hand HK to the PRC will not be there in this scenario: The PRC could deploy troops within hours and take over HK.
It would also be interesting to see how the three would adjust to become a large polyglot society that contains four major languages: Mandarin, Cantonese, Taiwanese, and English. I would say that it would be in the ROCs best interests to not militarize or politicize HK as well, to keep it as the unofficial "Gate of China", to prevent Shanghai from taking more of its thunder.