Nintendo consoles are notorious for featuring add-on ports that never have anything, well, added on outside of Japan.  The Nintendo Entertainment System had a port for an unreleased disk drive, the Super NES featured a similar slot that collected dust, and the Nintendo 64's EXT slot for the 64DD went unused in most places.  Even the Virtual Boy has an abandoned port meant for an unreleased two-player link cable.  Then there's the GameCube and its three add-on ports.  Do you remember the two serial ports that were meant for use with the limitedly supported broadband and modem adapters and the single parallel port that eventually found a purpose with the Game Boy Player?  Could that parallel port have been destined for greater uses?  Daniel Primed's Gamer Blog hypothesizes.

Back around the time the GameCube and it’s hardware specifications were first being shown off at E3 2001 Nintendo stated that the Hi-Speed port was basically a port for any console add-ons or peripherals down the track and that for the time being they weren’t sure what any of these expansions may be. I can’t remember exactly but I believe that the default assumption was that it would be similar to the Nintendo 64’s Memory Expansion port which later housed the Expansion Pak. That is, some sort of additional processing power for the console.

Eventually a year and a half later Nintendo released the Game Boy Player which was the only such peripheral to ever take advantage of this port. Which kinda makes me a little suspect as to what Nintendo were originally planning to use the port for. So I’ve thrown together some ideas of my own which I think would have been realistic of Nintendo and the hardware, feasible as well as useful for the consumer.

I've always wondered how these unused ports may have been better utilized, and while Daniel has some interesting ideas (miniature hard drive?  RAM upgrade?) I find myself wondering how feasible any of those ideas may have been.  I know there are some technically gifted readers out there, so let me put the question to you: what else could Nintendo have been planning for the hi-speed port?