Why are you so worried about that?! You assume that it would be for 4th Edition even though it's pretty clear Gary didn't want to lose control of his property. The only way I could see Hasbro ever being interested in that is for Gail to sell the work outright, something I highly doubt she would do. And I don't see Hasbro wanting to do that.
It would probably be a more likely scenario to see Castle Zagyg turned into a computer game. That would be a dream Gary always had--you wouldn't believe how hard he worked in the 1990s to get into computer gaming. And computer games can fund the development of print games (not the other way around).
But then again, I guess we'll just wait and see.