Funny related fact: Nintendo consoles before the Wii used to feature a "High Speed Port" (sounds like a slot for a modem at first but the Gamecube had one as well, in addition to its actual modem slot) - but not one of them actually got an expansion you could put in there. Even the N64 had one (on the bottom side) which was odd considering it also had a memory expansion slot.
I remember discussing the Gamecube port with my friends, we were so damn sure it would be used for a "graphics expansion pack" haha 
Anyone in here who knows more about this?
Edit: Apparently the Gamecube slot was used for the Gameboy Player. So I'm talking trash haha
(though "Serial Slot 2" was never used I think)
Edit 2: The N64 one was used for the 64DD, while the NES and SNES ones were used for the Satellaviews and possible CD-Rom expansions. You always learn something new! But damn it was way cooler speculating about these mysterious things... 
OT: If there's a screw we are not supposed to open it so yeah - no expansion slot for the Wii U.