6 player local multiplayer support (most HDTVs could split the screen 6 ways while still providing a larger screen area and more pixels than the TVsmany of us used to play games on).
Single disc online multiplayer; obviously this would have a limited number of gameplay modes and maps associated with it in games, but allow me to invite my friends to play games I own on my system. This would (likely) result in higher game sales for games that implement this well, because far more people would be exposed to quality games in a positive way.
3DS - Home Console connectivity.
Online store Homebrew developer kit. While I wouldn't expect Nintendo to open up the platform to everyone, and most of these applications should be developed using a high level interpreted language run on a virtual machine, a lot of value could be added to the system by hobbiests.
Integrated Avatar/Achievement system ... How I would implement this is producing the system with a built in Animal Crossing like game, allow developers to create achievements that have a meaningful in-game impact on this built in game (new items, activities, puzzles, characters, quests, etc.). Ultimately, the world of your avatar would start to reflect the experiences you have had while playing in that world as well as in the games you used that avatar in.
