Mario Tennis Aces will be seeing it’s character roster increase gradually as more and more characters will be added via updates. It has been confirmed that Koopa Troopa and Blooper will be joining as new characters to the game but it seems they are just the first of many. Okay maybe not many but Diddy Kong, Birdo and Koopa Paratroopa will be also joining Mario Tennis Aces sometime after September though it has not been specified when.
We will report more on this subject as soon as we hear anything more.
Source: Nintendo Japan