Daigasso! Band Bros. 3DS version! With improved song quality and creation tools and the ability to save, upload and download songs online into files that just go right onto your SD card. Ability to send a favorite song over streetpass would be nice, and a new, weekly spotpass song straight from Nintendo would be amazing. (or hey, how about they feature one user-created song from their servers each week, along with their own new song that week, and allow you to "subscribe" to either if you want to get them automatically with spotpass?)
Seriously, there's so much potential that an improved Band Bros could have. DBB is the best music game I've ever played by a long shot, and there's still enough it could improve on with the better hardware we have now. America never got Band Bros, which was a travesty... that's why it would be a doubly awesome thing to see a 3D sequel announced for all regions.
Meteos 3DS would be another great one. Meteos is my favorite puzzle game, right above Lumines and the Puzzle League/Tetris Attack series. It's a great way to fill that need for matching 3 or more same-colored blocks in a grid, but in a clever spin, the blocks literally rain down from above and you have to match them to turn the blocks to rockets to launch whatever is over them off the screen. The physics were perfect, the various planets each having their own unique play styles and quirks, the sound design being still some of the best I've heard in any game. The game is hard, built upon frantic action and quick thinking 100% of the way through, which makes other puzzle games feel boring in comparison. The various modes are also perfectly tuned to take advantage of the unique gameplay, action and difficulty level, and the unlockables keep you playing a lot of various modes and planets, keeping you playing and keeping it fresh the whole time.
The Disney version was an abysmal disaster, the android version is a shoddy, highly questionable port of the DS game, and the 360 version, while looking and sounding great, made an excellent argument for stylus controls, as playing with buttons was super slow and felt incredibly awkward enough to completely ruin the experience.
It's about time the 3DS gets its own killer app puzzle game. The Vita will have Lumines realized in gorgeous HD, and the 3DS should get the very best Meteos can offer, on the only hardware aside from Wii U that can pull it off.