DS Daily: What's missing?

We were at the GameStop, browsing through the old stuff as we tend to do, when we happened upon a copy of Heiankyo Alien. We were giving serious consideration to buying it, but in the end, the inconvenience of not being able to play it in our DS weighed heavily on our decision. So Heiankyo Alien sits unpurchased and unloved in a glass case, surrounded by copies of Mary Kate and Ashley: Crush Course.

Backwards compatibility may be one of the worst ideas in gaming, but it certainly does come in handy. Game Boy compatibility on the DS would make for a much friendlier device, and one with which we'd be even happier. What single feature do you desire? It could be anything that a previous handheld system has had, or a new feature.