It's weird to think about Nintendo joining the mobile market. iOS and Android are probably the Nintendo 3DS' biggest competitor. However, when they joined forces with mobile developer DeNA to work on the Pokemon GO! app they opened up their doors to a whole new world of gaming. A recent open board meeting with their investors revealed their plans moving forward with the first five mobile games being developed with DeNA. In keeping with their traditional approach to gaming for the masses, the first collection of games, starting with Miitomo will be free-to-play. Of course when the words "free" and "mobile" come together, micro-transactions are inevitable. Hopefully Nintendo has the salt to make sure they don't go overboard with these.
Not much is known about Miitomo other than it will be centered around social interaction, and will have integration with Nintendo's upcoming Nintendo Account cloud service and the My Nintendo rewards program. Miitomo is planned for a March 2016 release.
Source: Wall Street Journal