After the Heldeberg accident, the regulations concerning "combi" type aircraft were changed. I'm not familiar with the details, but those who are have told me it would close to impossible to certify a new combi aircraft under the latest regulations. I'm thinking retrofit (e.g. A380 combi) would be even harder since you couldn't design some of the stuff in.
All the combi's currently in operation (e.g. 747-400 Combi) were certified before the regulations were changed. I seriously doubt we'll ever see any more new build combi aircraft.