Looks like we have the motherload of content on some rumors Retro projects that were cancelled years and years ago, all thanks to the gang at Shinesparkers.
A gallery of concept art from Sammy Hall, a former Retro pre-production artist, has just been discovered online, and it contains all sorts of interesting art and blurbs from Hall himself. First up is a cancelled Zelda spin-off title, which is a rumor we've heard floating around for years now.
The game was in the works sometime during 2005 to 2008, and it would have focused on the origin story of the Master Sword. The game would have taken place during the bad ending of Ocarina of Time, and would play as an action/JRPG where you follow the last Sheik while Dark Gerudo are attempting their 100-year birth to Ganon. According to Hall, this project never made it past pre-production.
Along with that, Hall also shared art for a cancelled Boo project, which was tossed around during 2006 to 2007. According to some of the tidbits mentioned by Hall, it seems this would have been a project for a portable, most likely the DS. The art makes references to “possession powers” and “Broomies”, alongside a sketch of what looks to be a world map.
If you want to see more concept art for the Zelda project, you can see a number of images in Hall's portfolio here.