Metroid II: Return of Samus DX (rumored lost build of unfinished GBC remake of game; existence unconfirmed; 1990s)
Metroid II: Return of Samus is the second game in Nintendo's critically-acclaimed Metroid series. It focuses on the bounty hunter Samus Aran traveling to the planet SR388 to exterminate the Metroid species. In the end, she rescues an infant Metroid, an action which has major consequences throughout the rest of the series. As it was released on the original Game Boy, the game has no color and is entirely monochrome. Apparently, a colorized "DX" version for the Game Boy Color similar to The Legend Of Zelda: Link's Awakening DX was planned, and even received a feature in the German magazine Club Nintendo. However, nothing else about it is known.
Notably, the screenshot shown in the magazine shows a room that was not present in the original game, suggesting either that the screenshot is a mockup of an incomplete product, or that the room would have been featured in a new area similar to Link's Awakening DX's Color Dungeon.