While I would like NSMBU to be online (even though I probably won't actually play online), expecting Nintendo to be a pioneer in the world of online 4 player co-op action platforming is a bit of an ask for them. It's obviously not their priority and I don't think they will make it theirs until they see a multiplayer platformer (like a future Rayman, for example) go online seamlessly. Then they might put some effort into figuring it out.
Till then, it ain't gonna happen.
Till then, it ain't gonna happen.