yeah it definitely wouldn't be an easy thing to pull off, making a platformer work online is supposedly a lot hard to pull off then a racing game like MK, but other developers have tackled the issue before and have come up with pretty good results. Of course there have has been some awful results too.
I feel that having top notch connections is different issue here. I don't think we are anywhere close to a time where it can be assumed everyone has a decent internet connection, but that should not stop us from having online modes in games because a lot of us do have decent internet.
There are things that can be done to minimize people with bad connections too, like a simple signal bar icon, like MKDS can help or they could do what they did with Mario Tennis Open on the 3DS and make online region locked. some people won't be happy with that but it would prevent situations where two people are just too far away from each other to have a decent online game.