I tend to agree with Rob and FMIII that should this rezoning be carried out , it should be done with great care to avoid destroying too many prewar towers.
I would rather see parts of Manhattan further north, say the 125th street corridor massively upzoned, with appropriate transit improvements. Don't know if the MTA would be able to afford a new subway line along 125th though.
Extra long term, I'd like to see a new rail station on the caliber of Grand Central or Penn built in the northern part of the island to anchor a new business district. Maybe at the railyards at 10th ave and 207th?, with a complimentary subway line or two into the Bronx. Now I'm just dreaming...