What kind of Castle do you want?
Actually what would be most useful is a lcastle ine of brick and minifigure sets. Have arches, gates, flags, weapons, plates for battlements, etc. That way you would not be reduced to following directions for some ugly regular set designed so kids can build it easily. I want something with maximum customability, and pretty much no design........maybe a book of ideas and building stategies for people new to the MOC world. It would pretty much put designers out of a job, and would get rid of the need for all those sets like Harry Potter that lego had to spend millions on rights for, thus raising the price of alll legos for the common builder such as you and I.