The Official Pokémon site has recently updated with some further details on the upcoming Rumble U. When you begin a new game you must pick from either Pikachu, Snivy, Tepig or Oshawott before starting. You are then able to unlock and discover new Pokémon along the way. Abilities of Pokémon can be changed or upgraded by spending coins found throughout the game or the ability +/++ system.
The site says that there are various ways to unlock new Pokémon, including scanning IC Cards (an electronic train ticket used in Japan) to unlock Pokémon that can be used to help you out for one time only. The second set of NFC enabled figures have also been revealed. The set will be release on 25th of May for ¥200 and includes Eevee, Mewtwo, Croagunk, Zoroark, Litwick and Genesect.
Pokémon Rumble U will be released in Japan on the 24th of April, with a worldwide release date yet to be revealed. We will bring more details as and when they arrive.