Nintendo have added a host of new characters to Mario Tennis Aces including Birdo, Shy Guy, Koopa Paratroopa and Petey Piranha. You can unlock the characters by playing online and Nintendo have confirmed they will keep updating the game with more characters until next June.
There is also a new mode that will only be available temporarily called Co-Op Challenge where you and your friends can play online together to beat objectives. If you complete all the objectives you will unlock new special outfits and powers for your characters. Nintendo have committed to updating the game in the future and the Version 2 update will arrive on the 19th of September, so basically in 6 days.