Someone posted that as a prize for completing either the Pokedex or sticker collection (I don't recall which), you would get a Pikachu added. Is this true? I'm still 7 away myself, I haven't been able to find anyone local to fill in the monkeys and deer I'm missing.
Pikachu isn't actually in the Pokedex 3D - Pokedex 3D only uses the new fifth gen Pokemon (Black and White) - there is a persistent rumor about getting Pikachu for filling your Pokedex, but this is nothing more than a rumor.
There are various lists online of all the available Pokemon (like this one from Serebii), and you can also unlock the ability to view more Pokemon with the AR viewer at once:
200 stickers allow 10 Pokémon to be viewed with the AR viewer at once. 250 stickers allow 11 Pokémon to be viewed with the AR viewer at once. 328 stickers allow 12 Pokémon to be viewed with the AR viewer at once.
Other than that, though, there isn't anything else to unlock, and there is definitely no Pikachu. There might be, if Nintendo decides to update the application in the future to include more "classic" Pokemon from previous generation games, but for now, no, this is nothing more than rumor.