There was a proposal to convert the Princess to nuclear power, which maybe why they were cocooned for some time, in the hope something would come of this.
Had the Princess been successful, the next step would have been the Duchess:
Giant transport project from 1950's, page 1
Sad to hear that the Fantasy of Flight is closing. I really enjoyed the visit a few years back, with the sight of the Fieseler Storch in the air being the icing on the cake.