Really, though, I'd like to see something expandable. Something there for one purpose: Game expansions. A la Ms. Pac-Man.
There's several possibilities, but the idea would be that one device slot would be for the game, and the second would actually be the priority device, it's code is loaded first, and directs the first slot's code. Ms. Pac-man, I repeat, was done this way, and it was essentially what the 64 DD was planned to do, but it would mean game expansions would be possible. Super Mario Galaxy 2, for example, could be sold as an add on, since the engine, and so much of the data used would be on Galaxy, it would be cheap and efficient just to release the sequel on a small expansion SD card device, or whatever method.
If you don't understand, think Sonic 3 and Sonic & Knuckles. Actually, throw in Sonic 2, as well. Connecting Sonic 3 and S&K together meant you could play S3&K, allowing more characters and stages. As a bonus, S&K took Sonic 2's stage data, and put in Knuckle's character specifications and art, creating a new twist on a classic game. If Nintendo allowed at least two ports, it could potentially mean that S&K compatibility between games. While it's unlikely to happen, it would be a unique experience to play as Mario, with SMG's camera, through a Metroid Prime game, wouldn't it? Well, this would take minimal effort, and the actual ability to do so would only have to be implemented in one of the games.
An additional benefit is obvious: Why would someone sell their games if they become greatly cross compatible with future games? You would effectively eliminate a major portion of those resellers if you extend value of their games, and this would be a cheap and effective method.
It's a pipe dream, but it would be awesome, regardless.
Edit: Come to think of this, I might not be too far off. The DSi was originally planned to hold two DS cartridges. I couldn't fathom what reason they'd have for that, but maybe that was the plan? Nintendo ultimately decided that size and appearance mattered more on a handheld platform, but with a console, as BnM said, you want it to take up a certain amount of shelf-space. Since the Wii is GC 1.5, I'd shell out money for a GC 2.0 that had games primarily on SD cards, or some small media, if it allowed for the games to be compatible with each other in a serious expandable manner. Add in DVD functionality, upgrade the processors a bit more, and I'd be sold. I'd suppose it would almost reach the 360's raw power, but the big feature would be in the expandability options
