welp, time for me to step in and learn the young'ns about gameboys and n64's...
There was a device called the 64gb transfer pak. All you could do with this was transfer data from a gb game like pokemon to pokemon stadium. The idea was that you could train and level up characters from games like pokemon or potentially other rpgs, then transfer the stats of your now-more-powerful character into the n64 game. The device was a bust though, since no game other than pokemon stadium used it. I'm not sure, but I think the thing came packed in with the stadium game. The 64gb PAK is the one that plugs into the controller (in the same place as the rumble PAK, or memory PAK).
There was also a device called the GB hunter. I don't remember the exact name of the device, pretty sure it's hunter, though. This thing plugs into the cartridge slot, and was basically an emulator. It has its own cartridge slot where you would plug in the gameboy cartridge. This thing sucked. It couldn't even emulate sound, so instead they just play this annoying music over and over. Since a gb image doesn't use up the whole tv screen, you could change the border from just black, to other crap. You could also change the color scheme/setting that was used when "colorizing" old monochrom gb games. None of this was enough to make up for not having the sound. I think it supported gbc, not sure.
Making an adapter to play n64 games through the gamecube is totally pointless. Used n64's are easy to find and cheap. An adapter would probably cost more than a used n64. They go for $30 now. And since you'd still have to find the games and buy them, there's really no point. I mean, I could see wanting compilation discs of nes, snes, and n64 games for the cube, but an adapter?
What might be cool would be for an entire game series to be put onto one disc. So you could have all the Super Mario games on one disc and see the progression or evolution of the series, and of the characters. And for series where the stories from one game to the next actually make sense and are consistent, you could have the entire history of that game's characters and worlds. Nintendo could call these compilation discs their "Legends" line. All Zeldas on one disc? I'd buy it in a heart beat. I know, this has probably already been suggested by other fanboys, right?