The Japanese website for Pokkén Tournament was updated with a hint at a new Pokémon fighter being announced for the game on Friday, January 15th. The post revealed that the new character's Japanese name will include the katakana syllable "ラ", or "ra". This could mean any of a long list of Pokémon to be added to the eclectic cast.
To wildly speculate, this could mean Pokémon with mega evolutions like Slowbrow, Sableye, Aggron or Swampert. It could be Vanilluxe! Or Stoutland, Darkrai or Metapod. Since over 100 Pokémon fit the description, it'll be tough to guess until Friday.
Pokkén Tournament will be released for Wii U in Japan on March 18th, and should come to the west in Spring 2016.