Nintendo studio Brownie Brown helped make Mother 3. It also, quietly, made a Professor Layton role-playing game, available only in the Japanese and American versions of DS game Professor Layton and The Last Specter.
It's called Professor Layton's London Life, and while it was a "mere" unlockable in the Japanese version, in the US edition it'll be playable as soon as you get the game home.
Which you'll probably want to do; this is no diversion or easter egg, it's a fully-fledged role-playing game that will take you dozens of hours to complete (it boasts "over 100 hours of gameplay).
Plus, being a Brownie Brown game, it's got a very similar aesthetic to the Mother series.
Professor Layton and The Last Specter will be out on in the US on October 17, and in Europe on October 25. Bizarrely, despite being set in London, the European version won't include London Life. Sorry!