It wouldn't be hard if nintendo created a dedicated server(s) for Metroid later for multiplayer online. "One Player" just means "One player per console". Which would make sense seeing as how it's a very extensive FPS and requires a lot of screen real estate.
I could definately see a patch in the future that adds multiplayer online available. The patch talks to the game like this:
-> Take maps (already on game disk)
-> Take characters, allow different colors for different players (samus, enemies, etc)
-> Patch provides code for intializing the extra feature and code for the rules of the game.
Given, unless nintendo releases a hard drive they wouldn't be able to do much with it, just a few lines of code. BUT, if they did have a hard drive, they could definitely add multiplayer in the future. Just like Xbox Live had downloadable maps for Halo. Not hard.
However, I doubt many would be interested if it was the patch only (my first suggestion) because it just isn't extensive enough. There wouldn't be anything like capture the flag or special weapons and there wouldn't be a map editor (or special multiplayer maps for that matter). I am, and have been, wondering why nintendo doesn't release a hard drive or even third party support for one. I'd love to buy an external hard drive for my Wii! They are cheap!