Recently I became a proud owner of Grand Theft Auto for the GBA and although I only played it for about 15 minutes, it appears that I finally stumbled upon a GBA's "killer app".
As far as I can tell, there is no "killer app" for the Nintendo DS.
However, when I think about this, there are a couple of suggestions that come to mind that would make me buy one instantly.
Unfortunately, I know they will never be made.
1) Fallout (1) for Nintendo DS. Pure digital sex. Maybe make it overhead view instead of isometric because theres only so much real estate that can fit into 256x192 resolution.
The touchscreen would be perfect for management of the dozens of buttons and stats.
2) ELiTE for Nintendo DS. Yes, there is actually an ELITE for Gameboy Advance, but its an unofficial project that requires the use of flashcart, and it uses "original" vector graphics.
ELITE for Nintendo DS could use at the very least plain shaded polygons, in addition to the touch interface.
If you don't know what ELITE is, think Privateer 1
3) Warcraft 2 for Nintendo DS.
Advance Wars can go kiss my keister.
"The submarine can only fire from water at this specific unit which in turn can only fire at this specific unit but only if the weather allows and if the Moon is in the retrograde".
Give me Warcraft 2 or give me death !
... wait, just give me Warcraft 2.
What would be your ideas for a DS killer app ?