I think Boeing would be foolish to go ahead with with larger 747 derivatives at this point. It seems like anything Boeing could produce just couldn't compete with the A380 which aready has an enourmous headstart.
It is unfortunate for Boeing that Airbus has the market cornered with the A300 Freighter. It is cheap as dirt. The A300 line is paid off, and any plane they sell is pure profit. 767-200 can't compete.
Airbus opened a can of some serious woopass on Boeing in the past decade. I am sad to say Boeing is falling by the wayside.