There's never going to be a sliver bullet game for 3DS that's going to drive the platform to record sales, but then there never was for DS either. If you take time to look back at DS in the US you'll find it was really a gradual increase and the result of a cumulative effect of moves with hardware and software.
The DS had pitiful sales in the US in 2005, like record lows not seen since the days of Dreamcast. The turnaround started slowly, in August 2005 with the release of Nintendogs and a much needed $20 price drop on the system. That was followed into the holidays with bundles, new system colors and key games (Mario Kart, Mario & Luigi 2, Animal Crossing). The trickle of key games contined into the first half of 2006 (Tetris DS, Metroid Hunters, Brain-Age) building up to the DS Lite, NSMB and BBA launching in May/June. From that point it was strength to strength with the rest of 2006 and 2007 bringing tons more colors, Yoshi's Island 2, PokéDungeon/Ranger, Clubhouse Games, Mario Vs DK 2, DKR, Pokémon DP, Brain-Age 2, Zelda PH, Mario Party, etc. There was really no one sigular moment where the platform turned around from "doomed" to "instant success" though, it was a gradual build upwards and a cumulative effect of software releases, with explosive boosts around the holidays and hardware revisions.
3DS really hasn't been that dissimilar so far. After a slow start (though not nearly as dire as DS was), Nintendo started the push with Zelda OOT3D and the eShop in June, followed with the historic $80 price drop in August, followed with key holiday software (PokéBlast, SM3DL, Mario Kart) and new colors, continued into the first half with more key software (Kid Icarus, Mario Tennis), building to the launch of 3DS XL and NSMB2 in August. They'll follow this for the rest of the year and into 2013 with more colors, bundles, Luigi's Mansion 2, eShop Pokéstuff, Layton 5, Paper Mario 3, Fire Emblem, Animal Crossing, Brain-Age 3D and undoubtedly a lot of other stuff. No silver bullet, but a gradual build amd cumulative effect, maybe not as strong as DS was but definitely enough to keep the platform viable and growing at the very least.