Back in the 2000s, Retro Studios apparently worked on Mario and Zelda games that were based on the Boo and Sheik characters. That is what Shinesparkers is reporting, based on concept art they have received from former Retro Studios pre-production artist Sammy Hall’s online portfolio. The concept art was from the pre-production art phase of the two games, back when Sammy Hall was still working at Retro.
The concept art from the cancelled Zelda project, featuring Sheik, is from 2005 to 2008. Apparently, the art provides a “fun origin story of the Master Sword” that is set within the timeline of Ocarina of Time’s bad ending. The game, which never made it past pre-production, would’ve been an action/jrpg title that “would have explored the journey of the last Sheik while Dark Gerudo are giving their 100-year birth to Ganon”. As for the Boo project, there isn’t as many details. The art is from 2006 and 2007. Hall mentions “tiny stuff for tiny handheld resolutions” in one of his sketches, suggesting that the game was going to be on the Nintendo DS. A world map was being considered, as well as “possession powers” and “Broomies”. You can see an image gallery with some of the concept art down below.