Eh, here are some of my thoughts about each character and the possibility of them for being the Champion/E4 Members. Spoilers, obviously.
Anna – My guess is that she’s likely a member of the E4 or even the champion. It’s pretty obvious that something is odd with her, and Shade even knows that she’s not an average girl. Plus, so far she hasn’t revealed much about her position in the Reborn League, especially how the scene in the orphanage feels as if Anna's position was being intentionally hidden. So I’d wager that she’s ranked pretty highly as all Gym Leaders have been revealed at this point.
Cain, Fern, and Victoria – I’d rule out Victoria since she’s Kiki’s replacement at the academy, so she doesn't really have time to finish the badge quest. Cain’s out as he’s right behind the protagonist (Cainisim intended), so I doubt that he’ll manage to challenge the champion before we do. Fern is ahead of the protagonist in the badge quest, but aside from trolling us there is pretty much no reason why he’d be made the champion.
Heather – Earlier on, I’d say it would be likely, especially having a Salamence as her main. Though it doesn't really seem that she is the champion anymore. She may be a member of the E4, but that chance is pretty low.
Blake and Cal – Not much info about them is given, though Cal doesn't seem as he holds a high position in the League. Blake on the other hand seems to have some sort of power there, so I’d say that he’s indeed a member of the E4, or even the champion.
Ame – She’s more of an administrator rather a trainer. Plus, self-inserting yourself as a someone as a champion is generally frowned upon and considered bad writing, so I doubt the real Ame would pull something like this.
Sigmund – Highly unlikely. He seems to be an antagonist. Plus the player can already battle him, so I honestly doubt if he may become a member of the E4 in the future.
Laura – According to Charlotte, she was removed from the position in EP 13. I have a theory that it may have been through El’s interference to move Bennett up in ranks, which leads us to…
El and Bennett – El has some power in the Reborn League to promise Bennett a position in the E4, so he may as well be one of them. Laura’s sudden dismissal coinciding with El’s promise hints that he may have kept his promise all along.
Arclight – Possibly. We only know that he’s in charge of the Reborn Nightclub, nothing else.
Lin – Lin already plays a major part being the leader of Team Meteor, not to mention that she was once the champion, so I doubt she’s still in the League anymore. Plus, if Shade’s someone to go by she’ll probably die before the finale is reached.
Solaris and Sirius – Doubt it. They are already holding the villain card pretty tightly, so it wouldn’t make much sense to give them a second part in the story.
Silhouetted Girl – There’s still at least one character that has yet to be revealed. Considering how late in the storyline we are, she will likely be holding a very strong presence in the main story.
So my guess for the E4+Champion is: Blake, Benett, Anna, Silhouetted Girl, and either El or Arclight. Maybe Laura instead of one of the above if the story twists again. Likely with Anna or the Silhouetted Girl being the Champion.