How much more can they do before they lose the type-rating? If you are talking redesigning flap systems what relationship is that to the certificate?
The 747-8 has an entirely new flap system (and a largely new wing) relative to the 747-400. It can be done - it just a matter of how much Boeing is willing to spend when it's unlikely it'll be a big seller. Even they admit the -10 is really only a stop-gap measure until the MMA comes on line.