If they released something like an official emulator that was also a Nintendo market.
You would use the emulator/market app to browse their retro back catalogue and download one of their touch optimized retro games that would play within the emulator.
Have group score boards where applicable or maybe ability to share clips of game play through it.
Social aspect with it to see who else is playing the same games and maybe talk/shout to each other through the microphone, sort of bring the old school gaming feel back when you all grouped round a TV shouting to now being over the internet.
Well I do like what @AudaciouslyAwkward mentioned in having some Kirby games in there...I think that those would be fun. I also agree with some of the other users in that racing games seem to be a natural fit for a smartphone game. The Yoshi puzzle games would also be good, and now that I think about it Nintendo really has the advantage for thse types of games that you can play on the phone, so I am pretty excited to see where this goes from here.
As I've mentioned before I'much of a mobile or smartphone gamer, so the only game I'd possibly play would be Mario Kart. The only games that I think work well on the smaller screens is the racing games, but having said that, I can see even Mario Kart being a struggle for some people due to the speed and actions of the gameplay involved.
In all honesty, I'm not sure what extra exposure Nintendo would get from releasing their games on the mobile platform, and while I suppose any bit of exposure helps, I'm not really sure it will be worth the effort involved to be honest, but I'm sure Nintendo know more about that than me.