I think one of the first responses had it right, now that there is a game pad with a screen in it, DS and 3DS games on the wii-u preferable ala an adaptor like super game boy or the game cube game boy player for advance games, as well as digital download which they plan on starting with regular DS games soon from what I was understanding.

a small hardware add-on wouldn't piss people off that already have games they like plus it could be cheap, it would only be a means of getting the info from the DS cart to the system for the system to run, it in it self shouldn't do any processing.
another thing that would be interesting and I think both systems have the ability to do it is being able to use a ds/3ds as a controller for the wii u. or maybe that's how they could communicate in order to get ds/3ds games on the big screen...