Bianca, Cheren, Blue, Cynthia, Misty, Brock, Giovanni, Lance, and Steven are said to be returning. What their role is, however, is still unknown. Images of Coro Coro Magazine featuring each of the said characters have been revealed.
Bianca takes a role in giving the protagonist their starter Pokemon, as a favor for Juniper. Apparently she is indeed Juniper's assistant.
Alder also appears early in the game, and asks the player to follow him. To do what, however, is yet to be said.
Cheren returns as a Normal type Gym Leader.
Apparently there is some mentioning of N, as well as the appearance of at least one of the Seven Sages.
Brycen- still keeping his position (supposedly) as Iccirus City's Gym Leader, as well as being the FATHER of Homika.
Some Gym Leaders from previous games can be seen and battled through the Pokemon World Tournament located within the game.
...I'm getting more excited and giddy the more I hear. <3