The exact fact that benett is evolving is why i think he isn't part of the elite four.
As i think that most of them should have been already established.
We already have DJ Arch Light.
I expect one of the gym leaders to be the gym leader from the past, who got stuck in the grand stair way. I see him helping in defeating Team Meteor, and then being promoted to Champion. With him being from the past, he should have some Pokemon that no else in the game does.
Heather, maybe as a champion, even though she has dragon types we have no confirmation that the Elite Four are going to be based on types. Considering that we have 18 gyms, i'm expecting the E4 to have somewhat balanced teams which are particular to their field effect.