Nintendo's early infrastructure has its roots in GameSpy tech, it would make sense for them to outsource that again, and Steam would be a great choice -- but I feel to hope for such a thing is to invite epic disappointment.
Speaking of Epic, they - and Valve - are exactly the kind of third parties Nintendo should be courting. UE3 has been used in frikkin everything... I actually think they should look into getting an enhanced version of UE2 for 3DS made, a version that would really take advantage of the Pica 200 shading capability and 3D features of the console. A new GTA on a Nintendo console would be nice, and it sounds like they're working on that, but I worry that GTA is old news these days. They need to ensure that they have robust FPS and dudebro engine support. A Bungie game would be a coup if they could secure something like that. As cock of doody is still flavour of the month, they should probably liase with Activision / Infinity Ward / Treyarch on what they'd like to see too. How about speaking to Blizzard and getting a true World of Warcraft experience in the console sphere?