No that's not a bra size!

The 64DD was an add-on for the Nintendo 64 that was announced in 1995 and planned to be released alongside the N64 in 1996, but due to multiple delays, it was released on December 1st, 1999. Needless to say, it flopped and it's most likely the worst-selling Nintendo system of all time selling a little more than 15,000 units. during the 4 year period of development, a lot of games were planned for it but were never released. so far, we know there are 17 64DD games that were either cancelled, reworked into different games, released, but on a different system, or simply just faded away. I'll be going through all of them.

Cabbage; date: 1999-2000

Cabbage was going to be a Tamagotchi like game utilizing the 64DD's timekeeping capabilities. Games such as Animal Crossing and Nintendogs supposedly used mechanics from this game.

It was supposed to be shown at Spaceworld 2000, but never appeared. It was cancelled after that.

Diablo; date: 1997

After Nintendo purchased the Japanese distribution rights to Diablo, they were going to release it onto the 64DD. It most likely would've been a PC port and not it's own game.

Emperor of The Jungle; date: 1997

Emperor of The Jungle was going to be an adventure game based on the manga Kimba The Lion. It was shown at Spaceworld 1997, but due to some problems occurring during development, it was cancelled.

Famicom Classics Vol. 1; date: ???

Not much is known about this one, all we know is it was going to be a compilation of Famicom games (obviously).

Gendai Dai-Senryaku: Ultimate War; date: 1999

This game was going to be developed by SETA Corporation and was a turn-based strategy game that was going to utilize the 64DD's online capabilities. It was shown at Spaceworld 1999 and after that, was moved to being a cartridge game, removing the online multiplayer, and then it was cancelled.

Mario Paint 64; date: 1997

This was going to be a sequel to the awesome SNES paint tool, Mario Paint. This was announced in the magazine Dengeki Nintendo 64 in 1997. This was most likely an early version of Mario Artist: Paint Studio, but there's nothing confirming or denying that theory.

Desert Island 64; date: 1996

Desert Island 64 was going to be a survival game where the player was on (wait for it...) a deserted island. You'd have to farm, breed, and colonize the island (kind of like Minecraft!). It was shown at Spaceworld 1996, but was cancelled soon after due to financial reasons.

Fire Emblem: Maiden of Darkness; date: 1997-2000

This game was mentioned in an interview with Shigeru Miyamoto in 1997 and was formally announced in 1998. it was reworked into Fire Emblem: The Binding Blade for the GBA.

Mother 3/Earthbound 64; date: 1994-2000

Mother 3 started on the SNES in 1994, but was then ported to the 64DD were then ported to cartridge format. at this stage, a playable demo was shown at Spaceworld 1999. After that, it was put in development hell and then cancelled in 2000. After that, they took the story for Mother 3, and developed the GBA version. Footage does exist for this game and it can be found on youtube.

Rev Limit; date: 1996-1998

Rev Limit was going to be an arcade-style racing game, originally developed for the N64, it was ported over to the 64DD and then cancelled in 1998. In 2016, a collector known as コアラ's GAME SHOW found the prototype for the cartridge version of Rev Limit and filmed gameplay footage of it. 

Simcopter 64; date: 1997-1999

Simcopter 64 was a port of the classic PC game. It was shown at either the 1998/99 E3, but it was received poorly and the US version was cancelled and after delays of the 64DD, the Japanese version was cancelled as well.

Super Mario Disk Version/Super Mario 64DD; date: 1999

This one was simply a port of Super Mario 64 that was enhanced in some ways. Thankfully this was found and dumped and you can find it on 64dd.org

Super Mario RPG 2; date: 1997

This is also easy to describe. It was an early version of Paper Mario that was originally going to be developed by Squaresoft. it was shown at Spaceworld 1997 but was cancelled due to legal issues with Squaresoft.

Multiple Mario Artist Games; date: 1999-2000

There were 8 Mario Artist games developed for the 64DD, and only 4 were released. Those unreleased titles were:

Mario Artist: Game Maker

Mario Artist: Graphical Message Maker 

Mario Artist: Sound Maker

and then the weirdest one... Mario Artist: Video Jockey Maker

US 64DD; date: 1995-2001

I've forgot to tell you, the 64DD never made it to the states, and it was believed no prototype unit existed. Until Collector, Youtuber, and Atariage Member Metal Jesus found a US unit. It came with a blue disk which I believe he dumped.

This took me way too long to write so I hope you enjoy this!

sources: https://lostmediawiki.com/Mario_Artist_(unreleased_Nintendo_64DD_games;_1999-2000), https://lostmediawiki.com/Super_Mario_RPG_2_(lost_pre-release_version_of_"Paper_Mario"_Nintendo_64_role-playing_game;_late_1990s), https://lostmediawiki.com/SimCopter_64_(lost_build_of_cancelled_Nintendo_64DD_remake_of_PC_game;_1997-1999), https://lostmediawiki.com/Super_Mario_Disk_Version_aka_"Super_Mario_64DD"_(found_unreleased_Nintendo_64DD_port_of_3D_platformer;_1999), https://lostmediawiki.com/Rev_Limit_(lost_build_of_cancelled_Nintendo_64_racing_game;_1996-1998), https://lostmediawiki.com/Mother_3/Earthbound_64_(lost_build_of_unfinished_Nintendo_64/64DD_video_game;_1999), https://lostmediawiki.com/Fire_Emblem:_Maiden_of_Darkness_(lost_build_of_cancelled_Nintendo_64DD_tactical_role-playing_game;_1997-2000), https://lostmediawiki.com/Desert_Island_64_(lost_build_of_cancelled_Nintendo_64DD_role-playing_game;_late_1990s), https://lostmediawiki.com/Nintendo_64_Disk_Drive_(partially_found_unreleased_video_games_for_Nintendo_64_add-on;_late_1990s-early_2000s)