I totally agree with the idea of fan-designed castle sets. There have been other great fan sets between the Siskind Blacksmith shop and the recent Star Justice/Space Skulls:
The two [url=http://guide.lugnet.com/set/10025]Santa Fe train car sets designed by James Matthes:
[url=http://guide.lugnet.com/set/10190]Market Street designed by Erik Brok:
[url=http://guide.lugnet.com/set/10183]Hobby Train - which involved several train builders, IIRC:
[url=http://guide.lugnet.com/set/5526]The three sets in the microscale Factory line came from a contest judging builders designs (including Nathan Sawaya, Nathaniel Kuipers and others):
I think that various Mindstorms AFOLs were involved in the development of Mindstorms NXT. There are also AFOLs who have gone on to become LEGO employees, so there are sets designed by them in that role - Jamie Berard, Misterzumbi etc.